500 Handmade Books is a great visual reference, although it doesn't really go into detail on how to build. For that, Cover to Cover is one of the best I have found.

I like Keith Smith's books. There are lots of them but they all demonstrate lots of techniques (the Non-Adhesive Binding series, at least). I find them very inspiring.

Former collegues bought me a copy of The Thames and Hudson Manual of Bookbinding, which is serious & detailed, assuming you're not after artsy-craftsy bookbinding, but proper (ahem) stuff.

Proper is relevant. Maybe the "proper" binding for a booklet is a French-folded folio with a 4-hole Japanese stab bind, because the booklet is about the history of Japanese prints? Or maybe it's a 2 inch thick journal and it needs a good ol' coptic stitch? :)