moving from tex-pdf to a reader-friendly format?

authoring target: web pages, kindle, iBooks, nook. + secondarily

the first two editions of my free corporate finance book were created
in pdflatex. about 800 pages, 200 (pdf) figures and tables, and 800
(latex) equations. I wrote clever generic tex macros throughout, some
are "quasi-functions", other just "variable name standins".

I wonder whether there is an authoring solution for my third edition
which would allow me to create the book in kindle/ibook format (with
reflowable and resizable versions), and web-visible format, too? it
also needs less perfect than pdftex but reasonable book print output.

finally, to afford consistency, I would like a semi-programmable
system like LaTeX for authoring, so that I can define my own functions
and macros, etc.

finally, I desire one novel DRM feature (although I want all my books
to be always free), is that I want them to expire after 4 years. I
issue new editions ever 2 years, and I don't want my too-old versions
to hang around forever. for one, I correct errors in them. now, the
expiration method doesn't have to do a mission impossible
destruction. even a constantly annoying warning at every page turn
woud be enough that harrasses readers constantly would be enough.

what is a good authoring system for my needs?