As for which project to use... The issue with book management is that it's exponentially more complex than other media due to the number of dimensions a book can be on.

Author metadata alone can be problematic - some books are published under different names in different countries, some books are co-authored but published under all variations of the possible combinations (author 1 or author 2 or both, and that's if there's only two authors).

Language as a dimension usually means the same book is actually a different variant. This also applies for series info.

Then there's the issue of metadata quality. Unlike with TV shows and movies, where either IMDb or TheTVDb etc. can be used because generally all of these potential sources are good quality... books don't really have a central database, because unlike with the aforementioned, language as a dimension does affect the release, and can't be easily treated as the same entity as different language publications will have different IDs... So if you have a database of US books, that won't apply to anywhere else in the world. Of course GoodReads and HardCover are trying to fix this but you're still running into issues like API usage limits etc.

Overall, making a book download and management system akin to the rest of the Arr Suite is a major, major undertaking that requires major discussions not just within the project but also spanning external services to come to an agreement on which approach is best.