This is fantastic for undergrads! The undergrad lectures collected in the Feynman Lectures on Physics are enough to make any sane person question why the hell they're bothering with a physics major. The level of complexity is honestly grad level if you expect to read it with full comprehension. Luckily enough for the rest of us, this tidy little volume exists as a helpmate and confidence booster.
Collected within are four lectures that weren't part of the main sequence of lectures collected in the much celebrated introductory physics survey course from Caltech. The introduction to these lectures is more than enough to give the struggling student some confidence back again. Apparently, a bunch of the smartest kids in the country also had problems following Feynman and either performed poorly or felt so unconfident about their performance in the class that these problem-solving lectures and review sessions for laymen were crammed full of people. Every physics student can probably sympathize.
Here's what's great: Feynman and some of the other professors who ran the discussion sections put together a series of devious problems that are a lot more nuanced than they at first appear. Some have multiple solutions that show the versatility of the physical laws covered in your basic freshman survey series and to see them all worked out with step-by-step explanations from one of the greatest science teachers in the world makes lights turn on even for advanced students. The problem solving is far from dry, and the reader will take heart that Feynman himself humbly and self-deprecatingly admits to making several mistakes in these rather "trivial" problems. There is also an appendix loaded with supplemental problems for you to try. I think I'd recommend this single book over just about any other supplemental problem/solution guide out there for people who are either struggling with calculus-based physics or want to push themselves and test the depth of their understanding of the material. Give yourself the Caltech experience and see if you could have hanged with the best of the best.