A book useful to learn lattices (discrete groups) Does anyone know a good book about lattices (as subgroups of a vector space $V$)?

These notes of mine on geometry of numbers begin with a section on lattices in Euclidean space.  However they are a work in progress and certainly not yet fully satisfactory.  Of the references I myself have been consulting for this material, the one I have found most helpful with regard to basic material on lattices is C.L. Siegel's Lectures on the Geometry of Numbers.
