If i can - where should i store it?
Mb its real with some other mods?

Yes, all images you have in resources in the config folder can be used.
That is, you can put a PNG image in your config folder like this:

config/
    books/
        resources/
            textures/
                image1.png
        book1.xml

and then you can reference it as "gbook:image1"

Oh forgot to say, if you are making a resource pack for the book you can just have the images in the resource pack, and then you can use whatever namespace the resource pack is in, "gbook" or otherwise.

What am i doing wrong? Q_Q
(i'm trying to make resourcepack and add my own cover for book (48x48))

• I copyed "demobook" in same folder.
• Renamed it "bestiary".
• add "gbook:books/bestiary.xml" ] in book.json
• Changed mcmeta to 6 (i'm on 1.16.5).
  //from this moment i can see new book with the same name in the game:
  

then i find this issue #40 and tryed to make new cover:

• Creating new folder "textures" in <assets/gbook>
• adding my own picture 48x48pix in this folder
• changing in bestiary.xml book title="Bestiary" cover="gbook:textures/bestiary" fontSize="0.6"
  -got this...
  

It is trouble with picture or i made smthing wrong?

Btw if i will change default.png in mod folders, on my own - all will be fine:

UPD:
If i will try to use this picture on page - it will show pink-black squares (texture missing)

I fix it! (not for resourcepack, but it works xD)
So, if u want add any custom picture in this mod (1.16.5) use this inissue guide:

This works ony if u creating new books in gbook folder. If you created other folder for own book - use it. (c)Cpt Logic

I want add covers

• add it in Guidebook(your version).jar\assets\gbook\textures\covers folder in mod repository
• add link on your custom picture in .xml book with: cover="gbook:covers/ custom picture name without .png"

I want add my own pictures in books

• create new folder in gbook\textures\. For example i call it images, so now i have Guidebook(your version).jar\assets\gbook\textures\images
• put your best .png in this folder, so u can link them from .xml with: <image src="gbook:images/ your picture name without .png" tw="16" th="16"/>
• add your picture size in last 2 parameters (tw - width, th - height)

Ghz, sry cuz bothering you with stuped questions, but this awesome mod really need detailed wiki or better -- videoGuide!
I believe, one day, someone will create it!)

Uh you are not supposed to edit the .jar file! That will not work if you are making a modpack to share with others via curseforge or such!
I haven't had time to look into your issue yet, but it should work from the config folder, or using the same paths in a resource pack.

I'm making server with it own launcher, so its not a problem ;)
But yes, i will try in next few days do the same with resourcepack. I think i understand now, how it must to be, while changing .jar file.
Thank you for your attention!