I have all those books. The clinical avian medicine and surgery by Harrison etc. is quite technical. You can find an abridged version (a heavy paperback) which I bought in school and I still find useful. That’s from about 2000. The unabridged hardback version I bought when I misplaced the paperback.
I think “the parrot in health and illness” is still useful information. You mostly need to keep in mind that info about vaccines and treatments will be outdated somewhat. But otherwise there’s a lot of good info. One runs into the problem of outdated info in any layman’s book. Seems to me that only up to date textbooks and journal articles will have completely ‘right’ info.
Another older book I think I still have, and like, was called “feeding your pet bird” by Petra Something. Helped me plan balanced and less allergenic diets for rescued plucking parrots. But from about 2000 I think.
Always question any info you see in a book that doesn’t seem right. Authors are not God and their book are not flawless.