1. Chapter 4 of Zhu Guantian's Calligraphy History of China-Volume of Sui, Tang and Five Dynasties.

2. Baidu Library has an article "On the Formation of Yan Zhenqing's Calligraphy Style", which can be found.

3. There is an online article "Historical Orientation of Regular Script", in which Yan Zhenqing is mentioned, and the styles of regular script between Xu and Yan are compared, which involves the evolution of Yan Zhenqing style. However, it should be noted that it may not be reliable to quote some newly unearthed works of Yan Zhenqing.

Find the rest yourself.

For reference only.