Because it's written in simple English, this book is useful for beginners.
Literal
Plain English [with-で] written-is [because-ので], this book [topic-は] beginners [for-に] helpful.
役立つ ('to be useful/helpful') describes the book's value to a particular audience. Notice 書かれてある rather than the more common 書かれている — the ~てある form emphasizes the deliberate human act of writing, treating the resulting text as an intentional artifact. Subtle, but worth feeling: ~ている sees the state, ~てある sees the agency behind it.