What are LaTeX templates for A5 booklets?

I usually use the geometry package, with a5paper option (and like 7pt font size). The use of a5 size is an excellent idea!

You could consider looking at the source of the The Not So Short Introduction to LaTeX, which you can either find on your machine

/usr/local/texlive/2012/texmf-dist/doc/latex/lshort-english

(or whichever language is appropriate)

Or else you can unzip lshort-5.01.src.tar.gz from http://tobi.oetiker.ch/lshort/ and you'll find lshort-a5.tex which you can use as a starting point.

Have a look at external: A5 Reports with LaTeX and external: Formatting documents for the iPad, maybe you can pull something from these examples.