Books can tell you way more than a movie can. They are different animals after all, speak different languages. Books can breath better, ruminate. For the Disaster Artist, this means letting the world of this story widen and deepen and what the hell-en.
Greg Sestero had a dream to be an actor but found that his dream was just not that easy to accomplish. Then he met Tommy, an eccentric rich guy with a strange accent and stranger outlook on life. Tommy "guides" Greg along, offering him a place to stay and a chance to share in the dream of becoming big starts. Telling the truth of the American Dream through a passion project neither of them were prepared for: The Room.
Told in a mixed timeline with Greg and Tommy meeting in one and flashing forward to the process of making The Room in the other, the story weaves a crazy tale of strange excess and denial. Tommy comes of stranger and more endearing in the movie made from this, with clearer motives. In the book, he's more of a mysterious figure with his heart not so much on his sleeve but maybe on his knee? Like patched somewhere down there so you can see it, but not so you think: yeah, okay, heart on a knee.
If you liked the movie, read the book. It's that simple. You'll get more crazy, more weird stories, more oddness. Strangely, you'll also get a little more heart as you see how these men not only became but stayed friends.