For example, you can try:
https://www.oecd-nea.org/janisweb/book/neutrons/
ENDF data is written in a very strict and reader unfriendly format (it was defined in the 60's and it is meant to be read by a fortran program, not by a human).
The latest version (2023) can be found here .
A mandatory read is the section (in the Appendix) about MT numbers. This is the integer that is assigned to any reaction channel that is mentionable in the ENDF files. Any self-respecting nuclear engineer will know for instance that MT=102 refers to the (n,gamma) reaction, or radiative capture, that is a neutron capture followed by the emission of some gamma rays.
A very interesting reading (still in the Appendix) is the section on resonance models: what they are and how their parameters are supposed to be used. The models discussed are:
Single-Level and Multi-Level Breit-Wigner
Reich-Moore
R-Matrix Limited
An attempt is under way to replace the ENDF format by the General Nuclear Data Structure (GNDS) which is both human-readable (xml) and extensible. You can find it here .
Resistance is strong 🤨: a lot of codes will need to be partially rewritten in order to take this new format into account. The ENDF/B-VIII.0 and VIII.1 have been released (also) in this new format.