In-game documentation is important, but spawning with an inventory full of guide books from random mods is an overwhelming first impression.
Therefore, I would like to suggest that you consider disabling giving the book by default and instead provide it in some other manner - for example, as loot in villages or as an easy crafting recipe.

Yes, I am aware it can be turned off, but I would encourage you to find more intuitive ways to teach players how to use your mod than with book items dumped into the inventory on spawn.

Just to add on to my second suggestion: while I do believe the runic system should be removed from this mod, I would like to see it split into its own mod and expanded from there, as it is a nice system and would serve well as an alternative/replacement for enchantments with a split off focus from simply swords.

As requested, I'd like to suggest some alternatives and possible solutions to the situation - some of which may be controversial, but I will encourage you to strongly consider them regardless:

The Problem

Since the beginning of modding in Minecraft it has been a common practice to put a lot of "helpful" items in the player's inventory when starting, as a way to introduce the player to the mod's content. This has always been a problem more than anything since mods are more often than not combined with other mods.
The number of starting items when playing with a larger selection of mods used to fill one's entire inventory, leading to overwhelm and throwing away most of what's given regardless of how useful or necessary it is.

These days, the situation is less severe, as the introduction of Patchouli has allowed modders to provide handy guide books in place of starting items.
Unfortunately the same issue is encountered as before; once several mods are installed the number of guides becomes overwhelming with the amount of information being thrown at the player on their first join.

Challenges

There are several issues that make finding alternatives to providing starting items a challenge:

• There is no "one size fits all" when it comes to explaining what a mod has to offer
• Relying on external information such as wikis can lead to confusion/frustration and players ignoring most of what's included
• Minecraft itself provides no in-game guide
• The modding scene has no established alternative to books (yet)

While these are certainly challenging to work around, they are thankfully not roadblocks, as a rather popular mod known as Create has managed to fully document all of their content without the use of books!
This does not mean Create's approach would work here, but it is a good basis of inspiration to show that there is hope for a future without starting items.

Solutions Preface

I'd like to preface this by saying it's incredible the amount of time and effort you've put into the guide book, that is not something I wish to discredit here.

Solution 1: Intuitivity

Firstly, I would encourage focusing on how your mod blends into the game and how players can encounter each of the elements along their journey.
This is actually one that is for the most part done pretty well already; the legendary weapons are found in random places, blacksmiths and such will have the mod's weapons etc.

That said, there are still some areas that could use improvement, particularly around the recipe book;

• The recipe book should list all the associated weapons when you collect the related ingot - for example, collecting iron should unlock all the iron swords/glaives/etc
• These weapons should be listed in the weapons category rather than misc
• Legendary weapon upgrades should be hidden until the associated legendary weapon has been found

With those improvements to the vanilla recipe book it would take the place of the basic weapons list in the guide book - one section down!

As for the runic tablets; vanilla actually provides a mechanic that would serve this purpose much better:
Smithing Templates.
Not much would change about the runic tablets besides their name and visual, they would just integrate into the game better as smithing templates and it would be intuitive how they are to be used as it's a vanilla mechanic.

Solution 2: Simplicity

Let me be frank; this mod does not live up to its simply swords name anymore. The need for a guide book to begin with is a sign of bloat and overcomplication in a mod that should otherwise be focused on providing a few fun sword-like weapons to play with.

I would argue that the entirety of the runic gem/tablet and weapon socketing systems should be completely scrapped.
Allow the legendary weapons with their unique effects to truly stand out, allow enchantments to exist (not that I particularly like the vanilla enchanting system). There is no need to provide an advanced system to augment already strong weapons.

With a simpler take on the mod's design a guide book would no longer be necessary - at least, not to the extent that it would need to be provided as a starting item.

Solution 3: Pondering (WIP)

I previously mentioned that Create's system would not be a good fit here, but it may yet be; once they make a dedicated mod for it I suspect pondering will become the go-to replacement for guide books.
Something to keep an eye on!

Conclusion

While I recognise that simplifying and making mechanics intuitive is nothing revolutionary (even Minecraft has done so with things like the igloo to teach players how to cure villagers) they are areas that do not receive as much attention as they should.

Providing a cohesive experience is key to escaping the guidebook limbo plaguing modded Minecraft, and I hope this gives you some ideas moving forward.
Thank you.

It’s a problem with Patchouli itself that its design expects modders to fill the player’s inventory with books.
They really should provide a GUI based guide with a menu or inventory button to access it.

Regardless, I do believe it would be most sensible to just make the book easy to find or craft and not include it as a starting item at all.

EDIT: They can’t easily add a config option for the starting item due to the way modern modded Minecraft works, hence the jumping through hoops with data packs.

Can I just give a shout-out to Patchouli Button?

Also, I don't see why this is such a big issue. Even if some want a guidebook when spawning in, others do not. At least having the option to prevent this behavior seems beneficial. Absurdly obvious, even.

Can't you just disable the Patchouli behavior?

Then, have a server config that says whether or not to give the player patchouli:guide_book{"patchouli:book": "simplyswords:runic_grimoire"} on first join.

Most of the mods I use that give the player a guidebook on join seem to detect the presence on PatchouliButton or something because with that installed they automatically do not also add a book to the inventory. One of them (Mythic Charms) doesn't seem to have detection and always gives the player a book even if the button is installed but that mod had an option in the config to disable giving a book on join. I checked, and the book is still accessible in the patchouli button.

I know Mario said adding a config option isn't easy but if most of them work that way it shouldn't be too much work, right?

Unfortunately that only applies to Fabric mods as PatchouliButton has no interest in making a Forge version.
It remains a big issue because users are forced in most cases to learn datapack creation just to get rid of them.

There is another solution though: instead of putting these books in the inventory, they could be added to the bonus starting chest.
This is a vanilla method to provide additional helpful starter items that is sadly glossed over by most mods - its only issue being that it has little presence on servers.

With that said, I stand by my statement that this mod has become too complicated for its own good if it needs a guide book to begin with, and would love to see it return to its “simply” roots by just providing more swords to the recipe book and unique swords to chest loot.

It is pretty complex. I guess it would be nice to split out so there's one mod with the weapons nice and simple then a supplementary mod which adds in the fancy bits like gem sockets and runic stuff and whatnot.

Question, I found the bit where the book is actually being added to inventory

{
"fabric:load_conditions": [
{
"condition": "fabric:all_mods_loaded",
"values": [
"patchouli"
]
}
],
"conditions": [
{
"type": "forge:mod_loaded",
"modid": "patchouli"
}
],
"criteria": {
"tick": {
"trigger": "minecraft:tick"
}
},
"rewards": {
"loot": [
"simplyswords:grant_book_on_first_join"
]
}
}

and I couldn't find in the documentation where the fabric and forge conditions are defined, but are there boolean operators available? Maybe it would be possible to use negative conditions like this?
"type": "forge:mod_loaded",
"modid": "patchouli"
AND
"type": "forge:mod_NOT_loaded",
"modid": "patchoulibutton"
I don't know if that's possible though, couldn't find it in the documentation, was my first time looking through.

Indeed, checking if a mod isn't loaded is possible:

    {
      "type": "forge:not",
      "value":
        {
          "type": "forge:mod_loaded",
          "modid": "patchoulibutton"
        }
    }

Though as a Fabric mod I'm not sure how that would apply. Perhaps other mods supporting it have an answer?

Hello! Sorry for barging in, but I’d like to share a datapack I created that disables granting a book upon spawn. You can download it from this link:
https://www.curseforge.com/minecraft/texture-packs/simply-sword-patchouli-book-remover

To make it work, you’ll also need a datapack loader. I recommend using Global Packs.

I hope this help!

Thanks for the info, however this issue is about adjusting the default settings of the mod!
I'm hoping to encourage a future where one can "Simply" drop in the mod and have it fit well into the game without extensive tweaking or datapacks.