A free, open source textbook that covers both introductory and intermediate material. It also introduces many topics that are usually missed by intermediate courses, like risk aversion, multi-tasking, land price gradients, and fishing to extinction.
This is the first book on introductory economy I read. Getting to know what are the basics on the field was great. As a engineer I could follow the calculus presented, but honestly I skipped many of it because it wasn't my main goal to learn as an economic science student. The book brings many concepts that are used daily, with great examples. Some chapters were nice, specially the last ones, like game theory, monopolies and antitrust, while others were boring, with too much graphics ans not very straightforward.
Can't rate this book comparing to others on the same subject, yet.