You can't go wrong with Still books.
Kenyon's
Lugers At Random was recommended to me as a first book by an experienced collector, and it was good advice. It is a bit dated but has a lot of useful information. I got mine for $45 used.
The
Mauser Parabellum 1930-1946 by Hallock and van de Kant can be had used with some looking, I got one for $225. The overall best value IMO are the Gortz & Sturgess 3-book set, around $300 new.
I was short on time and patience and started with the Kenyon book and a collectible WWII era P 08 from Simpsons. No regrets and I still have and greatly enjoy the pistol.
Legacy has a 1940 P 08 with a matching magazine right now that IMO is fairly priced for a dealer pistol.
https://www.legacy-collectibles.com/...azine-129.html