US ArmyThe Official US Army Sniper Training and Operations Manual: Full Size Edition: The Most Authoritative & Comprehensive Long-Range Combat Shooter's Book ... / TC 3-22.10) (Carlile Military Library)

The manual is quite a bit out of date , with myself being an ex Army Sniper , but it's a good starting point/ insight

Very in-depth book on understanding all aspects of being a sniper. A great read and super handy even if your just playing Call of Duty, my long range kills have greatly improved!