I would think there are a wide variety of beings that could teach you things. It depends on what you wish to learn about first, that corresponds to the particular entity you should deal with. For example, you dont want to be learning about healing or divination from someone or something that likes to be mischievous.
I learned quite a lot from my own familiar. Threw some things in perspective that I hadn't considered before.
Bottom line is, be careful who or what you decide to learn from. Do a little research beforehand and make sure you trust the entity you work with. The that's about my two cents here. Be safe!