How to link books of series together?
Your series table is compound. You need to split it up.

Series ( series_id, name )
Series_Book ( book_id, series_id, series_ordinal )


Now you can easily seek to the series_ordinal-1, series_oridnal+1 for the series. Storing the count() as number of books also seems redundant.

Genre and Theme are also pretty nebulous. That method of classification doesn't scale well, or work all that well with humans. Normally you want hierarchical tags, and cloud tags; otherwise, enter "is the Beatles pop or rock?"