The Leech lattice and other lattices
Richard E. Borcherds · arXiv (Cornell University) · 1999
This is an unpublished manuscript written in 1983-4. It contains several results about lattices (=integral quadratic forms) including the classification of the unimodular lattices in dimensions up to 25 and the construction of unimodular lattices with no roots in dimensions 26 and 27.