As @John Smithers says, have a summary at the beginning. I will add that make it very short, maybe 2-3 pages. I want to quickly read through the summary to see if the book is worth buying. I have read too many books with a 20 page introduction where the author tells of his motivations for writing the book, and I havent even decided if I want to buy the book.
I think the gold standard for engaging non fiction is The Black Swan by Taleb. He gives as much technical information as is needed, without boring the reader. He also uses stories to illustrate his point, and his style is humorous.
Stephen Hawking was told by the publisher for his book A Brief History of Time that every equation he put in would reduce his readership by half. While some say this is an exaggeration, I have found that too many equations turn people off. Its hard for a layman to make sense of Greek symbols. So try to keep equations to a minimum. Also, don't use big and uncommon words, especially technical ones. If you have to, make sure there is a glossary of technical terms at the end.
Finally- the same rules as for normal fiction apply. It should be easy to read, and you have to keep the reader interested throughout the book. If you feel something is breaking the flow of the reader, put it in the appendix. Give every reason for your reader to turn the page- dont assume that just because they read the 1st 100 pages, they will stick for the next 200.