There are five different types of tomes, military, fortune, provisions, inspiration and undeciphered. Each type of tome has 5 levels: I, II, III, IV, and V.
A level 1 tome will increase that hero's attribute by 100, a level 2 is +500, level 3 is +1000, level 4 is +5000 and finally a level 5 tome will increase an attribute by at least 10,000.
Military, fortune, provisions and inspiration tomes will increase the respective type's attributes. Undeciphered tomes will increase a random attribute by a set amount.
Effectively Using Tomes to Upgrade Heroes
The description of the tome usually underestimates the actual attribute increase for a hero, meaning some heroes's attributes will actually increase significantly more than stated.
This is because the paragon bonus of a hero affects how much power is added when a tome is used. For example, if you use a tome on Adira, who has a maxmium of 175% paragon bonus, you can get 1.75x more power from that tome.
The formula for hero power increase from using tomes:
Tome Base Increase + Total Attribute Paragon % * Tome Base Increase
So effectively, you want to use tomes on heros with the maximum paragon bonus for that attribute.
For undeciphered tomes, you want to use it on heroes which has the maximum paragon bonus for all attributes.