There are level requirements for learning skills/spells, which should be listed in the book's tooltip in red text if the character is at a lower level.

In the Abilities tab in the inventory window, the Skills section determines which types of skills/spells you can learn, and how many. Skills/spells are categorized by group (Man At Arms, Expert Marksman, Geomancer, etc) and level (Novice, Adept and Master); the level of each group determines how many of each skill/spell you can learn.

level 1 - 3 Novice
level 2 - 5 Novice, 2 Adept
level 3 - 6 Novice, 3 Adept
level 4 - 6 Novice, 4 Adept, 1 Master
level 5 - 6 Novice, 4 Adept, 2 Master

Learning level 1 of an ability costs 1 ability point, level 2 costs 2 points, level 3 costs 3 points, etc. If you have an ability from an equipment or talent bonus, that isn't counted towards the cost to learn the next level (for example, if you have Blacksmithing and Crafting at level 1 from the Scientist talent, learning level 2 of either will cost 1 ability point, etc).\


Note: Man At Arms skills are governed by Strength, while Wolgraff will probably be using Dexterity, Speed and Perception/Constitution. Make sure you are not learning something that will be penalized too much due to low strength (if it will primarily be used in combat).