This book is easy to read because it's written in plain English.
Literal
Plain English [with-で] written-being [because-ので] this book [topic-は] read-easy.
平易な英語で書かれている is a passive construction: 'is written in plain English.' ので connects the reason (plain English) to the result (easy to read). 読みやすい uses the ~やすい suffix meaning 'easy to do.' A well-structured cause-and-effect sentence.