I read very recently that Frank Mittelbach has a Latex3 user manual/developer's reference (not sure which) in the pipeline - in fact its said "... only 250 days to go!". Unfortunately now I cannot find where I read this most exciting news item since 2012. Certainly amazon.com knows nothing about it. Is it fact? When will it be published? How do I get hold of an advance copy?
No, currently, there is no learning manual available for LaTeX3 (L3). The only available sources are:
expl3.pdf-- this file covers mostly the philosophy of whatever is done in L3 and, importantly, explaining everything of the naming scheme.
interface3.pdf-- this is the reference manual for L3. For each package in the L3 bundle, it contains a chapter with the description of all public macros available in the package.
xparse.pdf-- this is the manual to
xparse, which is the recommended package for defining user-interface macros to your L3 code.
There are no other official manuals to L3, as of October 2015. Sure, some plans exist, one by the L3 Project Team, one by Enrico. However, for neither of them, as far as I know, the work schedule is clear, nor the release date.