Edit Aug 9'22: See next post for links to AppImages.
CoolReader is fast and small XML/CSS based eBook reader.
Grey has recently compiled and published a new version which works well under Fossapup64. It is always better to use a version compiled against ones' operating system. His version will support formats: FB2, FB3, EPUB, TXT, RTF, DOC, TCR, HTML, CHM, PDB, MOBI, Open Document files. But primarily I still run Bionicpup64 which doesn't meet the new version's glibc requirements, among others.
Fortunately, version cr3-3.1.2 available from here, https://sourceforge.net/projects/crengi ... olReader3/ will work, but perhaps not with all the above noted formats. Both 32-and-64 bit versions can be found there. I run them as 'external/portable' applications as explained here: https://oldforum.puppylinux.com/viewtop ... 20#1050720
Another option is FBReader. It may be available via your Puppy Package Manager, PPM.