Over the last 6-7 years, I have worked on numerous book projects for pastors, some you're familiar with and some you aren't. I'm not new to the work. (Matt was just the first guy to put my name on the cover; I've never ever asked for that recognition.) I have worked on bad books and good books -- which is to say, I've worked with bad sermons and good sermons. So the level of work it takes sometimes to turn a sermon transcript (the word-for-word script of what a preacher said from the pulpit) into a book chapter (a polished work of composition suitable for submission to a publisher) changes from project to project, but the process itself is fairly standard. Here's sort of how it breaks down.
1. Know what good writing sounds like and how to produce it. This is the first hardest step, and there are a billion little details involved in getting there. Lots of guys write books who have no business doing it. And anybody with a basic grasp of grammar -- and plenty of people who don't -- can take a transcript and noodle it around to look like a book chapter. But it will sound less resonant than the original sermon, not more. I call this the "toaster manualization" of Christian literature. You know, when you pick up a book by a famous Christian preacher and it doesn't sound much like their preaching? And in fact, it doesn't sound like a particularly interesting book at all? And you're like, but I love this guy's preaching! Why is this book so . . . bland? It's because some guy with good technical writing skills but little familiarization with the white-hot furnace of essential speech (to paraphrase Lewis, natch) has hammered the sword into a ploughshare. That's why. He made a toaster manual. If that's you, you can probably eke out a good living doing it. That's the good news. But if that's not you, or if that's you and you stared at my toaster manual lines like a dog at himself in a mirror, it'd be equally awesome if you decided to do something else. We don't need any more toaster manuals.
Pardon the manifesto. Ahem.
2. Underneath the ability to write well, however, is the foundation of good mechanics. Could you turn out a decent toaster manual, if you had to? Know yeself some grammer, duh. Are your word processor's spell and grammar check functions your first line of defense? Then this work is not for you. If you're a good writer, you can fudge on this a little bit. For instance, I like to make words up. And play with sentence fragments. Et cetera. If you happen to be an excellent writer, you can even convince the publisher's editor(s) that that's okay. It's art, for Pete's sake! But as my 11th grade English teacher Mrs. Woolley once taught us when we objected to Faulkner's getting away with all the same stuff she marked up with her red pen on our papers, once you know what you're doing, you can not do it now and then. But you gotta know the laws before you can play around with them. And knowing them means knowing which ones not to play with. Like, for instance: Its/it's. Their/there/they're. What commas do. What semicolons do. What exclamation points do. (That last one was a trick question. Don't use exclamation points!)
3. Take the transcript and delete any "church business" or prayers that appear in the introduction or conclusion. You may have to also delete the entire introduction and conclusion, because preachers don't often introduce or conclude sermons the way book chapters are introduced and concluded. The guys who tend to manuscript their sermons often do; but most guys don't. If the transcriber is a legalist, you'll also have to delete a bunch of "uh"'s and "um"'s. But most transcribers know not to include those.
4. Listen to the actual message, perhaps several times, to get an ear for the fellow's voice. This is less necessary if you're already familiar with his preaching voice. As an example, I only listened to a couple of the audio versions of the messages Matt preached that became The Explicit Gospel. I've been listening to Matt's preaching for several years now, so I hear him when I read him already. And some of the material in the book, as others have noted, is not new to those who are familiar with the theme of his ministry. A couple of other guys I worked for I had never heard or even heard of, so it was a lot more work. But knowing the preacher's voice will help you find the holy grail of this entire process: making the sermon read like a quality book while simultaneously sounding like the preacher, not the editor. So find some fairie dust or scrounge up some magic beans if you need to, but get into the preacher's voice as best you can.
5. Know what to add and what to subtract. We're talking editing now. Many good sermons have decent illustrations but books require better ones. And documented ones. This is probably the chief work of actual writing I do. There is also the fleshing out of existing points and the shaving down of unnecessary tangents. Preaching and writing are related arts, but they are also quite different. What may fly from the pulpit may not from the page, and vice versa of course. Sometimes you've got an extraordinary bit of preaching that sings extra well from the page -- I felt that Matt's "Jesus wants the rose" bit was like that. I didn't want to mess with that too much. It was near-perfect as it was, plus it had the added benefit of being so iconic, so widely-recognized that to mess with it seemed anathema. This is why it's an art, not a science. But you will also need to know the science of research. Sermons don't often come with documented quotes or lengthy passages from secondary sources. In most respects, this process is about making the sermon better -- not as a sermon, but as a message. You're allowed more length, so there's more room to develop engaging narratives in illustrations, defend claims with research, do more exegesis, and the like. You also have to work at weaving into every chapter a continuity that often doesn't exist in individual sermons, even if they're in the same series. Many times, the preacher's attempts at creating continuity between sermons in a series are just plain clunky in a book. The continuity is more explicit in a sermon; in a book you want to get into the "narrative" of the work. Even non-fiction books tell a story. That's what you want to find and ride like a hawk on thermal wind. When you get better at this, you can take a 4-page sermon transcript and a 14-page sermon transcript and turn them both into subsequent 20-page book chapters without losing the preacher's voice or adding too much of your own.
6. Don't ghostwrite unless you want to feel dead inside. I define ghostwriting as actually writing a book or most of a book for somebody who then contributes little more than their name to the project. Ew. If you're fine with ghostwriting, you're probably already dead inside.
That's a start, practically speaking. Go revisit Johnson's piece for the Scared Straight version. Phil says this work is "literally harder" than writing your own material from scratch. That isn't always true in my experience, but it is at least equally hard. And many times, the actual physical labor in this work is less time-consuming than writing my own stuff, but the emotional toll is heavier. As a writer, it is wearying trying to write in someone else's voice.