The For Collectors Only series of books on military rifles is a bit hard to find, and a trifle expensive. If you're interested in one particular type of rifle, it's the way to go.
I'm a big Mosin- Nagant fan. They are sturdy rifles, robust and reliable. They require little maintenance. Mine have had a lot of rounds through them over the years, and I can count the number of parts I've had to order in 30 years on one hand. That's a big contrast with some of my other old bolt guns.
Ammo is still easy to find, either commercial or surplus, although I'm sure the Golfer in Chief will figure out a way to tie Russian ammo in with his inane "sanctions" before long. If you don't have all you might need for the foreseeable future, this would be a good time to get some. I know AIM surplus and Southern Ohio Guns have cases in stock, and I believe I saw it advertised in The Sportsman's Guide as well recently.
You can still get Mosin Nagant model 1891/30 rifles at a reasonable price, though again it's hard to say how long that will be true.
However, if you want a good shooting, well built, well balanced rifle, try to find a Finnish Model M39.
The Finns fought the Russians just before World War II, and again during the War. Even though they had only a tiny army and virtually no air force, they administered some serious drubbings to the Red Army before they were overwhelmed by the sheer numbers coming at them. In the process, they acquired large numbers of Russian Mosin Nagant Model 1891/30 rifles. They rebuilt them and turned out an exceptional weapon in the process. The different types of rebuilds are all covered in the book above. It's complicated, and you literally can't tell the players without a program, so the book is very useful. In the 1980's and 1990's, you would occasionally see these rifles for sale , on offer from the big surplus dealers. I got five of them over the years, and I think they came from Samco Global Arms, Century International Arms, Southern Ohio Guns, Zanders Sporting Goods, and a pawn shop. They are all sweet rifles. Now, you might see one on an online gun auction site, but it has been awhile since I've seen them for sale in any quantity.
Two good books on the the Russo Finnish wars are:
You might also want to read about Simo Hiya.( There are different spellings of his name on the net)
This guy was a sniper in the Finnish Army, with 542 to 700 confirmed kills, depending on what you read.
I s**t you not.