Schaum's Easy Outline of Principles of Economics
is a great/cheap read for students looking for more than what their current textbook delivers. It can be a real time saver because it outlines the subjects and how to solve them without going into the fancy pictures and examples that the textbooks flood you with. For example, the section on supply and demand took me 2 minutes (of 2 small pages) to read, and the section on elasticities was only 4 pages. It is also the same size as a normal book, not a textbook. This book is a good option for studying because it gives you the same content but from a different point of view. The book contains sample questions with complete solutions for each section, and priced at under $10 new or and under $1 used, would be a valuable addition to any student who needs extra help in economics, but has a huge opportunity cost for their time (little econ humor there).
