I am planning an experiment about speech perception in hearing aids patients (a new field for me) and want to know about general approaches and strategies in hearing aids.
A nice starting point could be Harvey Dillon's book on hearing aids, and also nice overview papers written by Brent Edwards. This topic is a very big research area, therefore it is perhaps not a good idea for me or anyone else to try to summarize in a simple message.
It is very difficult to get commercially available linear hearing aids. Phonak still offers a limited number but the stock dates back many years. Their current product line along with Siemens, Oticon, Widex, Starkey, Unitron and Bernafon do not include a linear hearing aid. Some hearing aids can be programmed to be linear in gain, Starkey and Siemens and Phonak but sound cleaning tools will still be active.
personally I like
"Hearing Instrument Technology for the Hearing Health Care Professional by Andi Vonlanthen and Horst Arndt" as a very good overview of hearing aid technology and fitting strategies. However if you are looking at the different fitting strategies, i also suggest you read the how they were derived. I have been less than impressed by the methodology utilized by some.