It is not obvious from the tag name that books as about books about TeX and book-design is about creating books with TeX. Accordingly, books is frequently misused.
Should we rename books to something more descriptive? One possibility is books-about-tex, but that would make it strange for questions about books about, e.g., MetaFont. Maybe manuals?
To make matters more complicated, we also have documentation.
When I noticed the ambiguity of the books tag, I created the book-design tag for the sake of clarity. Further I talked about that in the tag wiki. I think that's sufficient, at least if we would correct a misused tag from time to time.
books is just a fine tag for books about TeX, LaTeX and related software, brief and concise.
I think documentation is good to refer for general documentation, most notably electronic documentation such as package manuals, not as good for real books which might be distinguished.
How about renaming books to reference-books?
I would also add a synonym from manuals to documentation, but that's because I can't see much difference between the two.
It's all very confusing, admittedly. I think we can remove books in favour of manuals (there are only 10 questions marked with books, and those can be retagged painlessly a few at a time in the course of several days), and keep the rest as they are.
The documentation tag is applied somewhat inconsistently currently, I'd imagine it to be mostly about package documents (hah!) that describe in detail provided commands, available options, behaviour and implementation details, most likely including source code.
Examples: float package, TikZ-pgf manual.
manuals is a good tag for any sort of higher-level user guide to a certain broader topic (e.g. typesetting mathematics, a TeX-derivative, or general introductory material), without describing every package, tool and command under the sun.
Examples: The TeX Book, The Not So Short Introduction to LaTeX, or ConTeXt, an excursion.
manuals tag is not a good replacement for books at all -- books is about books sold in physical form which you can hold in your hands, like The TeXbook, The METAFONTbook, or The LaTeX Companion. I would expect things in {manuals} to be potentially updated far more often, without regard to changes in pagination, though of course the aforementioned books would not exactly be out of place there (they are, after all, still manuals).