Still thinking about photobooks and adding to ones library... generally I check out Amazon and UK's Book Depository. The latter was once a source of excellent discounts but now I believe they are owned by Amazon and the big discounts are a thing of the past. What's more both these stores are generalists not specialists.
If you are looking for a knowledgeable, independent photobook store, I've always been impressed by the online operation PHOTOBOOKSTORE based in the UK. These people have good taste, knowledge and commitment and I really like their regular newsletter.
They offer rare and signed books and the information on individual books is of substance often accompanied with a vimeo run-through of the book. They also present recommendations and stock unusual lesser known books.
|From PHOTOBOOKSTORE's site - a typical book entry|
Then there is photo-eye which in the last few years has done more for photography and the photobook than most. But then I'm stating the obvious.
And last I've always liked LA based ARCANA BOOKS ON THE ARTS with photobooks and much more... well worth checking out. ARCANA has both a store front and online presence.