I can't say that I have a favorite reference book, but my most used is the Reader' Digest Complete Guide to Needlework.
I'm a huge fan of all of Elizabeth Zimmermann's knitting titles. I hesitate to recommend buying her as knitters either love or hate her books.
All Needful Things
"Knit on, with confidence and hope, through all crises." Except carpal tunnel.
- Elizabeth Zimmermann