Ask HN: Share your list of free e-books that're readable on mobile phone display

I am often travelling between cities and I like to read epub books on my mobile phone during the trips.

The problem is that this requires some research about what books to select, find or buy a suitable format that are readable on small screens.

In this thread, can we create a list of free e-books that are readable on small screens? A5/A6 size PDF files, .epub files, books made of collection of responsive HTML files, etc. are all good candidates. Books of all genres are welcome. Alternative reading material like papers (really hard to find ones that are readable on small screens), comics, etc. are also welcome.