John Biggar's The Andes: A Guide for Climbers
is a valuable resource (especially the latest edition), though his descriptions are usually brief. I haven't used the book for Patagonia specifically, but have in several other parts of the Andes and have found the information adequate and valuable especially for the areas of the Andes that have few or no other guidebooks available.
His difficulty ratings, times and route drawings/diagrams are very valuable, but be aware that sometimes his difficulty rating are a bit lower than some other sources.
I don't know why the book is currently so much at Amazon:http://www.amazon.com/Andes-Guide-Climb ... 0953608727
Try shopping around on other outdoor websites, but make sure it's the 3rd (2005) edition.
Chessler has it for a little cheaper. The book is worth it, IMHO.http://www.chesslerbooks.com/item/857-a ... ition-.asp
You can scroll through parts of the book below to see what it's like:http://books.google.com/books?id=BXLsT8 ... es&f=false
The Lonley Planet book Trekking in the Patagonian Andes does have descriptions of easier climbs, especially up north, but nothing technical. The descriptions and maps are detailed, but it's obviously more geared more for trekkers that might do a few climbs up volcanoes rather than geared for mountaineers.