From time to time I plan to write a beginner's book on LaTeX. But I'm not sure of this: isn't it better to wait some time more until a workable release of LaTeX3 reaches the market? I'm really paralized by this cruel doubt...
These is some discussion already about Help in choosing LaTeX2e books in light of LaTeX3 development. This of course looks at the problem 'from the other end', but a lot of the same points apply. Executive summary: we are simply not at the stage of having a LaTeX3 system which is close to being 'generally user ready'.
What's more important than LaTeX3 at the moment is solid coverage of the different engines: XeLaTeX and LuaLaTeX in comparison with pdfLaTeX. No beginner's book, IMO should ignore these any more. With respect to LaTeX itself, the basic stuff is all the same, but both XeLaTeX and LuaLaTeX provide simple access to system fonts, which for beginners can be very helpful.
The LaTeX2e approach is likely to remain relevant for many years, and it's extremely likely that a stand-alone LaTeX3 format will present a very similar 'front end' to the user as LaTeX2e does, especially in terms of things a beginner will need. So a good LaTeX2e book will serve as a good base for users both now and in the future.