-Another vote for John Walter's "The Airgun Book" . Packed full of useful info and some interesting opinion plus first hand tests. 1st and 4th editions are my favourites.
-D.R.Hughes "A Handbook for owners of the HW35 series rifles".
-Some of the old catalogues, not strictly a book I know, but the AKAH catalogues for example are excellent references.
I've got a copy lurking somewhere (ISBN ending 641 IIRC?). Slightly yellowed pages, as they all are, owing to the paper quality on which they were printed.
[pm me if it's of interest, I'll find what I paid for it and offer it at that price].