What is the mentor's (or teacher's) elevator pitch? Perhaps: "You don't have to spend 20 years reading books and thinking about this stuff and experimenting with what works: I can just tell you". Or, the tee-shirt might read: "I figured stuff out so you don't have to".
And there's a lot of value in that value proposition. It means that you, the student, are absorbing your learnings in the most efficent way. And that frees up your time and energy to put those learnings into practice. On the other hand, you might find that reading and thinking and discovering things independently is a pleasure in itself.
So, as you go along, you can at any time turn the dial on the mix of what you're being told and what you're learning for yourself. If you want to read ahead in the books and turn the lessons into more of a conversation, co-investigation, and debate, then that's a great option.
There are plenty of resources for self-directed learning and I recommend them all. Books, flash cards, sites and tutorial videos on the web, software (in particular score-reading training software). Also practice, explore, and experiment for yourself. Of these activities, practice is the one to focus on and to put most time into.
Books can, though, in places be a little unclear, ambiguous, even confusing. Sometimes two books tell two different truths about the same idea; sometimes that happens within a single volume. You'll think: is this a contradiction, or is there a piece I'm missing that would make them both true? On your own, you only really know after sampling enough different versions of the truth, talking to enough fellow-musicians, and seeing where the consensus lies or discovering the fact or insight that you were missing. This is experience, and experience takes time to acquire. A sense of deep understanding, the right connections, the right perspective from which to look at an idea or practice: these all take many and varied inputs and ingredients, and time, to bake.
But if you sit down and talk with someone who is experienced with the subject matter, who has been through the learning process, and who is willing and able to explain it clearly, then you can pick up this wisdom straight from the horse's mouth and cut out a lot of time and effort and difficulty. A mentor or teacher should be able to clearly explain things that might be unclear on a page in a book, removing all ambiguity and contradiction while leaving plenty of room for you to explore and express your individual taste and personality.
The book Mastery by Robert Greene is informative in this regard. In it, Greene presents a lot of evidence that most famous experts in their field worked with a mentor.