Twice since June I've had authors express concern that their books-in-progress will "fall into essays." Each time the author used the same phrase to describe what was perceived as a pitfall to be avoided. Through probing we learned that the concern was more structural than thematic -- there was in each case a sense that all of the chapters belonged together, but that they somehow might fail to cohere in some sort of logical or necessary way.
My answer is to step back and ask if any nonfiction book is really more than a collection of essays. Theoretically any chapter should be able to stand on its own with a reason for its existence, and a beginning, middle and end. There is a sort of ideal model that wants all of the chapters to be necessary, and that hopes they will link somehow to keep the reader interested and turning pages, but readers tend to dip in and out of books (especially nonfiction, but even fiction sometimes... have you ever heard of the mystery reader who gets the gist and then turns to the finish? They're out there!), and we know that many nonfiction readers prefer going back-to-front, whether in a magazine or in a book. Some magazines actually structure themselves that way, to reward the right-to-left reader who likes to start at what English-speakers would consider the back of a publication. And think of all the major world languages that read right to left. There is nothing particularly special about left to right and about the order of chapters as many of us in English-speaking universities understand them.
A book is what an author decides it is, and its chapters relate to the extent that the author declares it so. If two chapters don't seem to connect naturally, then it's the author's job to build a helpful bridge that demonstrates the link the author perceives. Just as transitions take us from one idea to the next in a well-structured essay, so chapter transitions lead us from one to the next, preferably with a bit of enticement so that we don't do what most readers do at one time or another -- drop in a bookmark and never pick up the volume again.
Keeping readers is a secondary challege to getting them in the first place. There are a number of ways to do this, and building engaging and easy-to-use transitions among chapters is surely one of the most helpful. Then it won't matter whether a book is at risk for falling into essays... for the infrastructure will be there to keep it together with much more than just two covers and some library stitching.