Hi Gilian
Suggest you get a copy of very informative book, by the BMA entitled The British Medical Association Concise Guide to Medicines and Drugs. My copy (2003) came from W.H.Smith, or you could try the publisher's (Dorling Kindersley) web site
www.dk.com. It costs around £6. There may now be a later edition than mine.
It contains in its 460 odd pages every drug available, under categories of use (Brain, Heart, Respirtory, Gastrointestinal etc) where the diseases/illnesses are described and the drugs that are used for them. Depression, dementia, AD are all included. There follows a long list of the drugs, with full descriptions of their use, side effects, dosage, dependency, and so on. A full index is at the end.
I have found it extremely useful (a) to find out what drugs are for what purpose, and (b) to look up prescribed drugs that I know little about.
Hope this helps.
Good luck and best wishes.
ps - I've just checked the DK website, a new edition is out and it costs £8.99 plus postage.