As Adam says, Kibby’s book is a must. It’s well-laid out and very easy to use. I found it a little difficult to track down on the internet, and there’s some confusion about older and newer editions (Amazon seemed to suggest the new edition was only a supplement).
I got my copy of The Genus Russula in Great Britain: with Synoptic Keys to Species, Kibby, G. (2017) for £21.95 from summerfieldbooks.com
And a microscope is a must, too.
All the best… Nick