If you've ever met a missionary from the LDS church, then they're working from this book. In fact, if you've read this book, it's almost certainly because you served as a missionary for the LDS church or were very close to someone who did. I imagine that the readership of this book among non-LDS is probably practically nil.
Anyway, I did not read it because I'm going on a mission (because I'm not LDS), but I do know more than one or two people who have served a mission. And this basically fairly boring book is probably the best insight a person is likely to get into the experience, without actually signing up to go on a mission themselves.
And well, I mean it's going over much of the same material that one would find in other LDS literature. It is, by definition, orthodox. Its authorship isn't even ascribed at a single individual. I don't doubt that much of it (perhaps all of it) was written by committee. It was interesting to see the messages of LDS missionaries broken down into systematic parts, as well as to see the material organized into lesson plans.
Much of the second-half of the book is given over to discussion of quite technical aspects of mission organization. These chapters will really test how much you're interested in the life of an LDS missionary, but if you get through them you will suddenly realize that you know about LDS missionary life than you will probably ever need to know.