Since it's written in plain English, the book is suitable for beginners.
Literal
Plain English [with-で] written-being [because-ので], that book [topic-は] beginners [for-に] suitable is.
適している is 'is suitable / appropriate' — the verb 適する ('to suit') with ~ている for the ongoing state. 初心者に marks the target audience ('for beginners') with に as the standard/beneficiary marker. The passive 書かれている inside the relative clause sets up the cause (plain English) before the main predicate states the result (suitable for beginners).