Complex sentence: Collet had read Dyer's book "Raiders of the Sarhad" while he was serving in the army, he returned to it when writing his dissertation for his Master of Arts in biography at the University of Buckingham.
Split into simpler sentences: Collet had read Dyer's book "Raiders of the Sarhad" while he was serving in the army. He returned to it when writing his dissertation for his Master of Arts in biography at the University of Buckingham.