Both Gundam SEED and Gundam SEED Destiny had 5-volume novelizations in Japan. Gundam SEED Freedom had 2. Only volumes 1 - 3 of the Gundam SEED novels had English releases. Here are Yzak-centric exerpts from them. Japanese ones were translated by me, and I use the term "translated" lightly as I ain't claiming they're perfect.

GUNDAM SEED (ENGLISH)
•   Volume 1: Divergent Strike
•   Volume 2: The Desert Tiger
•   Volume 3: The Country of Peace
•   Volume 4 (never released in English)
•   Volume 5 (never released in English)

GUNDAM SEED (JAPANESE)
•   Volume 1: Wings Passing Each Other (coming soon, see English release in the meantime)
•   Volume 2: The Desert Tiger (coming soon, see English release in the meantime)
•   Volume 3: Land of Peace (coming soon, see English release in the meantime)
•   Volume 4: The Descending Sword
•   Volume 5: Toward An Endless Tomorrow

GUNDAM SEED DESTINY (JAPANESE)
•   Volume 1: Angry Eyes
•   Volume 2: Wandering Eyes (no Yzak scenes)
•   Volume 3: Passing Gazes (no Yzak scenes)
•   Volume 4: The World Revealed (no Yzak scenes)
•   Volume 5: The Chosen Future

GUNDAM SEED FREEDOM (JAPANESE)
•   Volume 1 (no Yzak scenes)
•   Volume 2