I personal find Compression for Clinicians: A Compass for Hearing Aid Fittings, 3rd Edition by Theodore H. Venema is a joy to read. Not sure if that book helps DIYers though.
Isn't that available on Google Play Store and Apple AppStore?
https://play.google.com/store/apps/details?id=com.connexx.fit2go
https://apps.apple.com/mn/app/connexx-fit2go/id1121268496