I don't want this to become an apologia for my somewhat geeky interest in bookseller's catalogues, but the above selection does, I hope, provide some clue as to why they are such a great resource. For a start, there's a book catalogue for every subject. Just on this post we have Judaica, Iceland and early typography - just about any subject that grabs you will have had a catalogue devoted to it at some stage - although I confess that the Nordic book catalogue looks to me like nothing more than 143 pages of consonants and other letters with lines through them!
Catalogues are also fabulous bibliographical resources. The Jarndyce catalogue above on Dickens, whilst plain on the outside, it well illustrated throughout with b/w photos of Dickens's novels in their original cloth bindings and is full of disussion and information about variants, precendence and other slightly, I admit, nerdy bibliographical points.
Catalogues from the really big players like Maggs Bros can also be useful for biographical as well as bibliographical material, the catalogue above of autographed books, letters, manuscripts and so on is packed with sometimes lengthy quotes from letters which, the average researcher or student may never find but which might shed light on significant points about an author or other personages work or life. Anyway, enough apologetics...
Someone was kind enough to ask if I have an catalogues by Jacqueline Wesley, whose stock reflects a lot of the concerns of this blog. I don't. I wish I did. Gay bibliography is not a huge field but it is growing and I would always be grateful to be offered items of interest in that area - catalogues, bibliographies, reference books - please use the email link at the top right-ish of this page and let me know what you have and how much you think you would like for it!