both, but if i had to pick only one, photography. for the same reason that a manual for a digital camera (which is, in fact, a small single-purpose computer) would be in the photography section. conceptually, what you're doing with it is photography related rather than specifically computer-related.
This is the changing face of Photography. As computers are integrated into applications, the applications change but people don't look for instruction on using the application in the "computer" section.
The nature of Photoshop makes this a bit of a trick question because it is used by graphic artists, not just photographers. The question is easier if we change it to books on Lightroom, or to books like Photoshop for Digital Photographers which clearly belong in the Photography section.
This problem is one that is much easier to solve in a digital world than an analog world. In the digital world an item can be placed in or found in two places at one time.
Many (most?) stores have 2 book sections (or more) for books on photography itself. Books about how to take photos are found in the hobbies section - this is where photoshop books go if they aren't shelved under computer software. Books about famous photographers (including how they took their famous photos) are often found in the arts section. Books that consist mainly "of" photos are often found in another section entirely (coffee table books). Some of these are more about the subject in the photos but others are about the photos and photographer more than the contents of the photos - so where do you draw the line between niche market book about a photographer, and mass market coffee-table book of famous photos?