forge-1.10.2-12.18.1.2043-universal
Guide-API-1.9.4-2.0.0-40
BloodMagic-1.9.4-2.0.3-53

Here is my crash report:

http://pastebin.com/Lc4UpMjY

Still got the same problem:

java.lang.NullPointerException
    at net.minecraftforge.fml.common.registry.PersistentRegistryManager.makeDelegate(PersistentRegistryManager.java:694) ~[PersistentRegistryManager.class:?]
    at net.minecraftforge.fml.common.registry.IForgeRegistryEntry$Impl.<init>(IForgeRegistryEntry.java:67) ~[IForgeRegistryEntry$Impl.class:?]
    at amerifrance.guideapi.api.impl.Book.<init>(Book.java:16) ~[Book.class:1.9.4-2.0.0-40]

with latest Forge build (v12.18.1.2065) and latest GuideApi version 1.9.4-2.0.0-40.
Tested in dev/deobf environment, not sure about normal one.

Ok, solved the problem. If I create the book during INIT instead of PRE_INIT everything seems to work fine.

Please change the instructions here :)
https://guide-api.readthedocs.io/en/latest/createabook/basic/

No. Pre is correct init is not. Are you loading before gapi?

In the future, I will cause a more informative crash if a book is registered too early (before gapi hits preinit) or too late (during init or later).

Ok sorry, messed that up, totally forgot to set the load order.
Works fine during pre-init now, thanks :)

615504a

Reproduced, with 2044, just Guide API
http://pastebin.com/raw/T6uCQfWi

Probably a duplicate of #38

@tali713 That is actually a different crash. Yours is caused by #38.

Is this reproduceable?

Aha, I see. I still think the two bugs are actually the same bug, though it is peculiar that they raise a different initial error.

Once more with just blood magic and guide-api:
http://pastebin.com/KBCKw9Hk

All I can really say is to downgrade Forge until its fixed. I'll probably be doing a PR for it today

Please try with Forge build 2045.