Book of XP is consumable that can be bought from the market at a cost of 20, 30, 35, 75 or 100 Farm Cash each in the "Farm Aids" section. Cost depends of current level of player since it will give different amount of XPs. It also can be obtained for finishing some quests.
After using Book of XP player will instantly level up to next level. Any XP you had at your current level will wrap around to the next level. Before there was a limit of 1 Book of XP in Gift Box but it was lately changes so that you now can keep more than 1 Book of XP in Gift Box.
After using Book of XP player will instantly level up to next level but regardless to achieved XP in previous level it will be putted at begin of next level! So the best use is after leveling up and avoid using if you are close to next level.