[Hadrien]

Originally Posted by kovidgoyal

https://libprs500.kovidgoyal.net

Look at feedbooks.net for latex based pdf generation.

It's feedbooks.com ;-)
Well what you really need in your LaTeX template is:
\geometry{paperwidth=90.00mm,paperheight=120.00mm}
Any template should be fine as long as you create a PDF in this size.

We produce e-books on the fly with LaTeX for Feedbooks this way.

Check this URL too: http://www.osresearch.net/wiki/index.php/EBook