I believe there was a topic on training manuals a bunch of months ago. I would think that they would be a optional method of leveling skills/warding off skill rust. As is, hands-on work is the only way to learn. I personally think there should be more ways to learn.
The standard method of learning while doing is already implemented, however there is no real penalties to using no skill dwarves excepting with very valuable metals-dwarves caan't botch reactions and destroy reagents, they can't accidentally injure themselves, and they can't make 'shoddy' products (only no-modifer items). These should be possible disadvantages to using low skill dwarves to make items.
Another way could be a master-apprentice system where apprentices are assigned like hunting and war animals to a dwarf. They then share their task settings and possibly help out the master in other ways such as hauling reagents and products. This would be a faster way of learning, but would be impacted by the skill ratio of the master to apprentice as well as skills such as observation, leadership, and teaching.
The last would be by using manuals to study skills. This would possibly be the longest method of teaching, and would be dependant on the reading skill of the reader and writer. Manuals would be expensive and tiem consuming to make possibly, but their advantages would be that dwarves could stave off skill rust if you had a shortage of reagents, and you could train up dwarves a few levels to mitigate the consequences of low level-skill reactions.
In addition, books on poetry, fiction, philosophy, history, or religion could give happy thoughts to dwarves that read them. Ledgers could be physical objects written by bookeepers that get updated periodincally, with appropriate consequences for loss or destruction. Legends mode would be appropriately vague or specific on certain points depending on how much documentation was present of them. (this could help clear up a lot of clutter for creatures that never end up doing anything in world gen) They would still exist, but their wherabouts, names, or even physical descriptions or species might be unknown. For more documented cases, there might be more specific information, such as hair color or personality quirks etc.
Thoughts?