At the moment we have three tags concerning documents about TeX: books, manuals and documentation. The only one with a wiki is books, which says that it “is for questions which deal with books about TeX, LaTeX and other topics of this site.”
I just brought this up in chat and the suggestion was to keep books for “real” books about TeX (i.e. those with an ISBN) and merge the other two tags. Any comments on this proposal?
Additional points to consider are the usage of documentation for (rare) questions concerning writing documentation and how the tutorials tag fits into the picture (my take: “documentation” is more of a reference document, while “tutorial” is something you read only once).