Glen,
I agree that Photoshop unfortunately has become a defacto standard, BUT it's a VERY expensive solution for the amateur. There are other alternatives which are "similar" enough to make use of the various tutorials (and lot's of the plug-ins) without the cost....PaintShopPro for instance.
You still need an image acquisition software, AstroArt, Maxim, PI etc etc.
Steve's book is well regarded in the UK....
https://www.firstlightoptics.com/books.html