This isn't a bad book, but there is another booklet in the Today's Issues series which I believe covers this topic better. Ryken explains on the basis of scripture why Christians should not accept the validity of other religions. He doesn't do a bad job, but I prefer the approach the Rod Rosenbladt takes in "Christ Alone." In that booklet, Rosenbladt presents a more theological answer to this question.
If you want to explain to non-Christian why you can't accept other paths to God then Ryken's book may be for you. If on the other hand you are a Christian and want to see for yourself why Jesus provides all then the Rosenbladt book is better.