books ⇒ needs ISBN sounds good.
I don't know how the tags have been handled so far, but here's how I understand the three:
Documentation is about how the thing works, is typically written by the author or someone greatly involved with the project, and typically includes the source code.
A manual tells the user how to use the thing and can be used as a quick reference. Often structured closely along the structure of the thing. The
hyperref manual is a pure manual without documentation.
A tutorial can be considered a sub-species of a manual, more educationally written and process-oriented, typically longer than a manual; it takes the user by the hand and leads them through the process of learning how to do something or use the thing. It might be about several topics, like The Not So Short Introduction to LaTeX2e.
These three things are not clear-cut, a lot of "documentation" on CTAN starts out with a manual. A "book" could be any of the three and very likely a mixture.
Furthermore, I suggest the introduction of documentation-writing, parallel to package-writing and documentclass-writing.