Starting here, Reference materials for Pentax cameras, lenses. etc...
Official Ricoh manuals here:
Specific camera model Books from Yvon Bourque :
(I could only say, that those books are awesome in my personal opinion. I own one for K3II/K3 and another for K1, and it is lifesaver. A lot easier to read then official camera manual, with every option actually explained ! I didn't even know before reading it for example, that my K3(and K5, K7) etc... has awesome feature called Composition Adjustment in Live View that "emulates" Tilt Shift lenses, and helps tremendously with Architectural photography or helps create 3D effect on some photos. Also a lot of my "customization" options assigned to my personal taste/and experience for my Pentax cameras actually came to be after reading those books)
Also good website with SLR manuals: