EccentricTome version: 1.0
Patchouli version: 1.18.2-66
Forge version: 40.0.36
Minecraft version: 1.18.2

Combining a (filled) EccentricTome with another guidebook (in order to add it to the Tome) results in the previous guidebook being overwritten by the new one.

Might be specific to these guide books. I tested it with other ones (Tinker's Construct, XNet, and Extreme Reactors) and they seem to work. Thanks for submitting the issue. I'll investigate.

Note to self: maybe an issue with some of Vazkii's Patchouli books not being registered under the correct mod

Confirmed. Certain Patchouli guide books get registered to the same resource location, so the Tome thinks they're the same item.

@HalcyonSeas73, I've fixed this issue and published v1.0.1 to CurseForge. I tested it with Mystical Agriculture and it worked. Which mod did the "Network Guide Book" come from?

Network Guide Book is from Simple Storage Network, by Lothrazar.