Sierra Club pubications excepted, I have come across very few of Ansel Adams' phorographs on non-photographic book covers. This is not to say that they aren't out there, but I do find it puzzling; to the greater American public, Adams virtually was photography from the mid 1940's through the '70's. His images were punchy, often breathtaking, graphically compelling yet inoffensive in content, and ubiquitous. Adams was no art snob; even today, his landcapes may be found on posters, calendars, tote bags, mugs and screen savers (not to mention the infamous 1969 Maxwell House coffee can). I can only guess as to the reason for his absence on the covers of fiction, poetry etc. It may be an estate issue (read: expensive reproduction rights) but i doubt it. More likely, designers just don't see his work as embodying the 'geist' of their 'zeit'.
Covering Photography is a web-based archive and resource for the study of the relationship between the history of photography and book cover design. Read more about the project.