For those that want to explore more in depth specific issues, then professor Farnsworth's Treatise on the subject would be the reference text. For legal students in the common law sistem it gives a very useful approach to the subject matter.
Also consider the student edition of E. Allan Farnsworth's treatise on Contracts; the original was three volumes long, but the student edition is condensed to one. Farnsworth's discussion is more in-depth, wide-ranging, and denser than Calamari's, so I used Calamari to get principles clear and then turned to Farnsworth for elaboration.
Get both if you can; otherwise get this one first. That's my recommendation, anyway.
Used price: $50.00
The best part about the book is the problems after every section of cases. These problems allowed me to realize whether I understood that type of contract.
The worse part about the book is that the hornbook is a newer edition. So, although Calamari does an excellent job laying out the law, the labels (that tell you which sections in the hornbook to read) do not always correspond.
Used price: $100.00