i want to read the book which is written in English. More common. Shows that you are thinking about the 'state' of the book at the present time. You are describing how the book is, at the present time.
which was written in English. Suggests that you are thinking of the past, ie the time that the book was originally written.
The meaning is virtually the same. written is somewhat more of an adjective in the first one and distinctly a verb in the second. That is, the first describes the book as it is; the second refers to the act of writing it.