There are a few big book databases out in the wild like Amazon, GoodReads, Google Books (quite poor), Open Library (poor), other? Sadly none of the good ones seem have any decent license for re-purposing the meta-data.
As far as I know there are publisher co-owned companies that take payed for giving you access to the meta-data, but it's very regional and too high a price.
Are there any good approaches to making niche services that would require a good database of book-meta (titles, authors, cover images)?
To me it seems the book industry suffers from the lack of available meta-data, crippling innovation in this space.