Image processing books Gonzalez et al. vs Sonka et al?
I am not sure there are "best" image processing or computer vision books to buy. The topics covered can be very wide, and some can be better on some aspect (morphology, segmentation, denoising, etc.).

Some sites recommend a hanful of such books, and link to their draft versions, as in 8 Books for Getting Started With Computer Vision. Indeed, a lot of authors have a draft on a personal page. This is the case for instance for Computer Vision: Algorithms and Applications by Richard Szeliski.

CVonline: Vision Related Books including Online Books and Book Support Sites also points to classics, like:


D. H. Ballard, C. M. Brown; Computer Vision, Prentice-Hall Inc New Jersey, 1982, ISBN 0-13-165316-4.
A. Blake, M. Isard; Active contours, Springer, London, 1998, ISBN 3540762175.