What deep point of view comes down to is cutting any distancing words and just stating what the protagonist feels, thinks, and experiences. Mostly the culprits are words like "felt," "thought," "wondered," and sense words like "saw," "smelled, and "tasted." Using active verbs and avoiding author intrusions is also a part of this. The idea is to make it feel like the reader is living the story, not reading it. Writing deep point of view requires changing the way you think and write: you have to become the character, separate your own attitudes and reactions from the character's, almost like you're acting. At first it's hard, but when you've been doing it long enough, it becomes second nature.
The book says that it's not for beginners, but I found it easy enough to follow. It's a fast read with clear examples and useful exercises, so no technical stuff, if that scares you. Hall discusses using different points of view, picking the right POV character and avoiding head-hopping, filtering observations through the character's experiences and interests, giving the full sensory experience, the difference between male and female POV, using situation and mood as filters, writing character thoughts and emotions, the importance of keeping trigger and response in the right order, using similes for expressing backstory, how to describe your protagonist's appearance in deep POV, and the importance of body language. We also get a few sample stories. If you've read a few writing books, you probably know most of this already, but it's useful to have it all collected in one volume.
I found this book useful, but if all the things I listed in the last paragraph seem like kiddie stuff to you, might be that you're already writing deep point of view without knowing it. If you're a newbie like me, you might want to check it out. And it's worth noting that this book is part of a series. You can explore the other volumes on the writer's website.