I thoughally recommend Garrett Locomotive of the World (an updated version of The Garrett Locomotive) by the late lamented A E Durrent. This fascinating book lists all the Garretts that were ever constructed, giving details and comments as well as technical information. Unfortunately this is out of print, though I was luck enough to pick up a secondhand copy last year. Your County Library may have a copy however. This is a British published book however so I am not sure of the availability in the US.
Good Luck.