Hello, new poster here, but as a 19 year old girl working on my private license I thought I'd chime in.
My biggest difficulty so far has been engine systems since I have no prior experience with anything mechanical. Otto cycle? Carburetor? Spark plugs? And where the hell are those cheeky little pistons? (Best video I have found:
http://www.youtube.com/watch?v=LVqf7ArLBXs&feature=c4-overview-vl&list=PLCE7B71E76BE03C8A His other videos are also good)
I tried reading several books and even if the material made sense while reading I couldn't retain much of it. By far the most helpful book for me has been Rod Machado's "Private Pilot Handbook." His explanations are simple and written in plain English. Some of his jokes are corny but they really do help me pay attention when I'm reading. I highly recommend it when she starts studying for the written.
Personally I always thought airplanes were quite boring. They're just cars in the sky, nothing special. I actually got interested in flying when I read Elizabeth Wein's "Code Name Verity" about two young women in World War II. One is a pilot, and there's some flying involved. I'm not usually a fan of historical fiction but it was excellent, and when I saw the author was a pilot I wanted to learn more. If you're looking for a book for your daughter, I would recommend it. It starts a bit slow but it is really worth it.
I read it in June, went for an intro flight in September, started lessons in October and right now I'm having a blast getting ready to solo. Wish I had started sooner!