An earlier form of the Greek language which died out with the fall of the great Myceanean civilization (estimated 13-15 B.C.E.) Linear B has remained a mystery until fairly recently. Interested in ancient scripts, I happened upon this little treasure of a book in the Classics section of a used bookstore. The author, John Chadwick, is a key player in the decipherment of Linear B, and helped crack the code in the 1950s. Only 68 pages, it is a treasure trove of the basics, from its discovery on clay tablets, to its decipherment and relationship to other ancient languages.
One of the most difficult parts to follow, and rightly so, is a bit about how Chadwick cracked the code. Yet once I was on board with his methodology, the book was a very interesting and enjoyable read. Due to Chadwick's involvement with this language, he takes great pains to teach the reader about the many difficulties involved with deciphering and reading this script, notably the largest is the comparatively small sample size found (though archaeologists are finding more as time progresses). Included in this book is a list of the currently known symbols - the signs, ideograms, and numerals, making it a valuable addition to any ancient history buff's library, both amateur and expert. It is also a part of a series put out by the British Museum called "Reading the Past". As they can be difficult to find, I hope to discover more.
To learn about Linear B, see Wikipedia's article here.