Dover's new series of Phoenix Editions is dedicated to providing high-quality hardcover editions of books for which there has been a small but steady demand. So far, Dover has shown excellent taste in its choice of volumes, and I hope the mathematics community will support their effort to keep these useful books in print.
One of the highlights, for me, of the latest batch of Phoenixes is Felix Klein's Lectures on the Icosahedron. Despite the old-fashioned language and notation, this remains an important book. It connects the icosahedron to group theory (via its symmetry group), to function theory (via elliptic and modular functions), and to solving the quintic. The book is historically important (anything written by Klein would be!), but it's also interesting because there are very few modern works dealing with this material (Jerry Shurman's Geometry of the Quintic is the only one I know of).