Everyone thinks that there are only three (3) books in the 'Bourne trilogy', but there are actually four. The first three are written by Robert Ludlum, whilst Ludlum turned the fourth over to another writer, Eric Van Lustbader. I'm actually watching and reading them out of order, as it turns out, but here is the correct order:

"The Bourne Identity"
"The Bourne Supremacy"
"The Bourne Ultimatum"
"The Bourne Legacy"

And to further confuse matters, there appear to be a renegade film and book out there, one entitled 'The Bourne Conspiracy', and the other entitled 'The Bourne Secrecy', which as near as I can tell are simply alternate titles in different countries.

Additionally, there also appears to have been a TV movie/mini-series version called 'The Bourne Identity' (1988; Richard Chamberlain, Jaclyn Smith) and a gay parody film called 'The Bourne Identity Crisis' (2003). And it that weren't enough, I had a companion on my mission named Gavin Bourne. And apparently, there are quite a few Gavin Bournes in the world, including one that's in a rock band, and two or three that are vice-presidents of major tech companies. Very confusing.