. . . and another thing, along those lines. When I was a freshman in Engineering school, a H.S. senior asked me for help in permutations and combinations. That being Sputnik year (I'm dating myself, but then, it's OK cause I'm a guy) there had been a sudden upgrading of the math curriculum--and I had never studied that math topic either. But the difference was that year of maturity, and a few months' exposure to the discipline of closely reading the discussion in the textbook.And in this day and time the student should be working from a web site, and should have the ability to edify the site (actually as part of the homework assignment) so that the webmaster could incorporate the best ideas so the next cohort of students can get more out of it.
So the project of teaching a subject you have aptitude but not education in is merely challenging, not hopeless. I wouldn't want to count on doing it every day, tho . . . unless the student was eager and apt, in which case it might well be a highly edifying adventure!
The other thing I learned about learning from a book was that I had to try the assigned problem before I could really read the book. My inability to solve the problem motivated the reading, and I actually learned--whereas I generally could not learn much when reading without that specific incentive.