For most student the concepts are as difficult as the problems themselves. The Feynman Lectures fill a niche in teaching physics.
RICHARD FEYNMAN LECTURES ON PHYSICS VOLUME 1 PLUS
Plus the team that wrote the books are really eloquent and concise in their writing. Which leaves what accounts for the bulk of most textbooks to other textbooks. Short, it only uses calculus where you need it to fully explain the physics concepts. The contributions of The Feynman Lectures coauthors, Leighton and Sands (not to mention others) are overshadowed by Feynman's brilliance and popularity, but these people, who were in fact responsible for creating the book The Feynman Lectures on Physics, deserve a lot of credit, which they don't often get, for making Feynman so eloquent and comprehensible in print.
I just want everyone to be aware of who did what. So thanks very much for your kind words about the literary merits of The Feynman Lectures on Physics. Feynman's books would not read nearly so well if they were mere transcripts of what he said in his lectures. If you listen to the tapes while reading along in the books (which you can do at The Feynman Lectures Website at Caltech) you will see what I am talking about. All of his published books are actually written by other people. All the writing and illustration was done by other people - mostly his coauthors Robert Leighton and Matthew Sands, plus Leighton was assisted by a few grad. I am Richard Feynman's coauthor in the book Feynman's Tips on Physics, and also Editor of The Feynman Lectures on Physics, and I just want to point out that while Feynman gave wonderful lectures, he did not write The Feynman Lectures on Physics.