novels          

Both Gundam SEED and Gundam SEED Destiny had 5-volume novelizations in Japan. Only volumes 1 - 3 of the Gundam SEED novels had English releases. Here are Yzak-centric exerpts from them.

GUNDAM SEED (ENGLISH)
•   Volume 1: Divergent Strike
•   Volume 2: The Desert Tiger
•   Volume 3: The Country of Peace

GUNDAM SEED (JAPANESE)
•   Volume 1
•   Volume 2
•   Volume 3
•   Volume 4
•   Volume 5

GUNDAM SEED DESTINY (JAPANESE)
•   Volume 1
•   Volume 2
•   Volume 3
•   Volume 4
•   Volume 5