The source of instruction may be one of the following:
Some sub-skills within a category may not be listed in the category lore sheet. These are not directly studyable, even once the parent skill is known. They must either be learned from a scroll, a rare book, or through research (see research).
Many cities offer instruction in skills. Shipcraft , for example, is commonly taught in port cities.
The turn report lists skills taught by the location:
Skills taught here: Alchemy 
research, and is listed as `
Partially known' in the turn report.
Partially known skills: Improve ship rigging , 0/7
Inventory: qty name --- ---- 1 Old book  Old book  permits study of the following skills: Alchemy