J.W.S. Cassels 
Introduction to the Geometry of Numbers [PDF ebook] 

From the reviews: "The work is carefully written. It is well motivated, and interesting to read, even if it is not always easy… historical material is included… the author has written excellent account of an interesting subject." Mathematical Gazette "A well-written, very thorough account … Among the topi are lattices, reduction, Minkowskis Theorem, distance functions, packings, and automorphs; some applications to number theory; excellent bibliographical references." The American Mathematical Monthly