Good starter for under 20 bucks:
Zaloga's Osprey book is a nice intro, and it has some very good, Osprey style, illustrations that really clear things up a lot.
"Hitler's Greatest Defeat" by Col. Adair (who worked a lot with David Glantz) is a good study from the Soviet side
Also Neipold's "The Battle for White Russia" is good asd well.