From what I read:

Church And State In Ethiopia, 1270-1527 by Tadesse Tamrat. It gives a variety and detail informations on Amda Seyon I reign (consolidation of power, expansion, evangelization, etc). The book is considered the center of Ethiopian medieval studies.

For Amharic source, ንጉሥ አምደጽዮን – ጀግናው ኢትዮጵያዊ በመሪራስ በላይ gives a short but interesting account on Amda Seyon I reign.

Travels To Discover The Source Of The Nile, Volume 3 by James Bruce. It gives a detail account on Ethiopian rulers from Yekuno Amlak to Teklehaimanot II including Amda Seyon I. It was published in the 1790s and contains many errors but is still part of Ethiopian studies.