I don't even believe you necessarily have to read all the TP books. If, say, you don't like Titan you can skip book 2 and not miss anything. I think books 3 and 4 will have the most lasting impact in trek literature and would recommend them if you had to choose though I thought 1 was the best read.
We had a nice discussion on Typhon Pact, especially book 3 at the startrek.com forums. DRG III really helped me appreciate the book with his comments in the thread on the subject.