I spent a long time looking at their site and emailed them a few times. Each book is the same, but each adds more info. The first two use algebra, the last uses calculus. The Principles of Physics prepares for A/B and the Scientists and Engineers prepares for B/C.
From their website:
The textbook is available in three versions, designed to meet the needs of different physics courses.
Conceptual Physics
This version of the textbook is designed for students who are taking their first physics course in high school, or for college students who are not science majors. The textbook is algebra-based. Like Hewitt�s Conceptual Physics, it does include some equations. It might be considered a somewhat more rigorous version of Hewitt.
Principles of Physics
This version of the textbook is also appropriate for college students who are not science majors, and for high school students who want to take the Physics AP/B exam. It is a more rigorous textbook than Conceptual Physics but does not require calculus. To cite a distinction between the two textbooks: Principles of Physics includes topics such as the adiabatic process in an engine cycle, presenting equations relevant to the process, while Conceptual Physics does not. This textbook is similar in scope to Physics (Cutnell & Johnson) and College Physics (Serway & Faughn).
Physics for Scientists and Engineers
This is a comprehensive, calculus-based college-level textbook that is also appropriate for high school students who want to take the Physics AP/C exam. It includes topics such as the Lorentz transformation equations, power in RLC circuits and a calculus-based statement of Gauss' law for electric charge. There are many more derivations, including ones that employ differential equations, such as the derivation of the general wave equations and the field equations for an electromagnetic wave. This textbook is comparable in scope to Physics for Scientists and Engineers (Serway & Jewett) or Fundamentals of Physics, Extended (Halliday, Resnick & Walker).
All three textbooks are built on the same basic building blocks, in a modular fashion. We divide each of our topics into smaller "chunks," each a single Web page, which makes it easier to find and link to desired topics in a digital curriculum. The more advanced textbooks add more advanced topics and cover material in more depth, but the basic content is similar through all three versions.
We consider our textbooks to be at least equivalent to and probably supersets of the standard textbooks at comparable levels. This is not a claim that we are a higher quality text, which is for you to judge; it is simply a statement of fact that we cover a large range of topics that is not constrained by the costs of paper or the muscle-straining weight of many current textbooks. Of course, we are also able to go far beyond what any print textbook can do by taking advantage of computer-based multimedia animations, interactivity and simulations.
