Here’s what I said about Harvey Dillon’s book a while ago. The basics of hearing loss and hearing aid correction haven’t changed. DSP signal processing advances are just in the way the HA’s function to deliver sound recovery. I doubt any book is going to explain the “magic” of how any particular HA works as the OEMs are keeping that stuff to themselves as proprietary trade secrets, etc.
Then there is this nationally used text on audiology itself:
IMPORTANT NOTE ON KINDLE EDITION: Actually it turns out that the Kindle edition format isn’t so great for the pictures and the video available with the ebook. So in the end, I returned my Kindle edition purchase to Amazon and bought the etext directly from Pearson, which necessitates installing the Pearson app on a smartphone or using a web browser on a computer to read the book. But it does have some very nice illustrations and videos and sound media files, IIRC.
Martin, like Dillon, is a grand old man of the hearing/hearing aid world so as for Dillon’s work, his text may be petering out on updating, etc. The last edition was in 2018 - but then maybe such books don’t need to be updated that often?
A great free reference on hearing aid fitting, mostly about compression of sound loudness to restore hearing and how it works, is Starkey’s free downloadable PDF, The Compression Handbook:
@MDB very helpfully recommended the Dillon and the Martin and Clark books to me once upon a time.