The 2005 edition of Erwin Puts invaluable Leica Lens Compendium is now available as a free PDF download. This 223 page version does not contain any images, but otherwise is the complete text of the original edition. It is available as a free download with the permission of the author. Erwin released a new version of the hardcover book in August 2011, but it now appears to be out of print for the second time in 12 months.

This book, regardless of edition, is an invaluable resource, not only for those that are interested in Leica lenses, but also anyone with an interest in optical design and theory. Simply a must have research tool for all lens lovers.

If you're not familar with Erwin's writing you owe it to yourself to visit his web site. Thanks for the Christmas present Erwin, and thanks as well to LeicaRumors.com for bringing it to our attention.
http://www.furnfeather.net/books/pdf/llcforweb.pdf

source http://www.luminous-landscape.com/whatsnew/