Archive.com has the following books available for free (Should be legal):

• Ancient and medieval Ethiopian history to 1270 (Pages 164-174, goes into depth about the life of Yared)
• The book of the saints of the Ethiopian church, (pg 875-877 contains primary text info on his life)

The following websites are also useful:

http://www.tadias.com/11/29/2007/st-yared-the-great-ethiopian-composer/

https://www.styaredcuisine.com/about.html