High Frequency Trading, by Irene Aldridge - this book was quite good. It is kind of a technical how-to book about most all aspects of how to set up a serious professional high frequency trading system. It is a fairly technical discussion about a lot of topics ranging from simple things like how to calculate returns to correlation and autocorrelation techniques and risk management models, how to handle tick data, etc. I would recommend this book to people that have decent math skills and who already know a bit about trading...this probably isn't the best book if you know nothing about trading or how the stock markets function.
The Option Traders Workbook by Jeff Augen is also pretty good for learning how options work and how they are valued as well as how to structure different kind of options positions. Each chapter explains the principles it is based around, and then gives questions for the reader to work out, as well as answers to the questions.