William Goldman covers this in his book Adventures in the Screen Trade. It's a small part of a larger book so, no, you don't have to read the whole thing, just the parts that pertain. Where they are in the book, I don't remember, but I do remember this: Turning novels into screenplays is most often a question of what do I leave out? Here's why: You can't get it all in there. You can only capture the spirit of the work (and a few details, even if, yes, you must leave out some of the most remarkable ones). So you end up having to compress things. You compress time. You compress scope. You compress events. You compress multiple characters into some that never existed in the novel but make telling the story easier, more manageable. You compress in ways that keep the story true to the source because 120 pages of script will never contain what 700 pages of novel contained. (Or, in the case of The Godfather, which sits at my elbow, 162 pages of script versus 448 book pages, at least in a Putnam hardcover edition.)
There are other books covering the same process. Seems like J. Michael Straczynski has covered it in one of his.
But before you decide to pay someone to write the script, why not give that a go yourself? Buying a how-to book would be cheaper than hiring someone. And cheaper still would be checking a book out of your local library.
While you're at it, don't forget to do this, too: for an entertaining bit of research, pick a novel that's been turned into a movie and compare the two. Take note of what's been changed, and consider why those changes were made. Good luck, and have fun with it.