This is my favourite introductory book on Stoicism. Ryan Holiday and Stephen Hanselman collected 366 meditations that reflect the Stoic philosophy. Many are from the original philosophers, but they also include examples from people who embody Stoic Principles.
It slowly introduces you to the core concepts of Stoicism without overwhelming you. The commentary also allows you to see how the concept fits into a larger framework. It was this book that made me realize that this was a philosophy I wanted to explore.
Affordable book by Chuck Chakrapani. It is shorter than most and easy to understand, so is probably worth reading if you are undecided about whether you want to learn more about Stoicism. It covers many of the core concepts and relates them to other techniques that allow you to practice the Stoic lifestyle. I found that I wanted a lot more, but this could be useful for those short on time or who are uncommitted.
My favourite book that goes deep into the concepts of Stoicism.
If you need ideas for other books on stoicism, check out StoicBooks.com