"Brethren" and "Crusade" by Robyn Young are about the Templars/late crusades and are an ok read - though not fantastic.
"The Crusader" by Michael Eisner was a good book (Though i can't remember what period of the crusades it was set)
I'm just about to read "The Mosaic of Shadows" by Tom Harper which is the first book in a trilogy set during the first crusade (the others being Knights of the Cross and Siege of Heaven)- I'll let you know if its any good.
Thats about all i can think of, but if i come up with any more i'll add them later.