If you're looking for a less pricey English edition of Seneca's letters than the complete edition of the letters from Chicago University Press, this is the one to get. It contains the largest selection of the letters currently available in any English selection in an extremely readable and accurate translation and with a highly informative introduction. Each letter is prefaced with a precise and thorough summary and the original section numbers are preserved. The paperback version is a perfect gift to someone who doesn't now much about Stoicism yet and is willing to give it a closer look. For these reasons I highly recommend the paperback version of this book.
A word of warning, though. The ebook version of the book is pretty badly produced. First of all, the text is very poorly formatted if you use the standard "flowing text" mode of the Google Play Books app. Furthermore, since each letter is not its own chapter, it's very hard to find a specific letter and also hard to know which letter a given search result is in, if you use the brilliant search feature of the Google Play Books app. The complete edition of the letters from Chicago University Press is vastly superior as an ebook (but remember to check the prize carefully at both Amazon and Google Play. It's usually a lot cheaper at Google Play).