What kind of inner monologue is represented in your book? Are you a first-person writer, where the narrative essentially is the inner monologue of the protagonist, or are you third person, where we are outside, with only the occasional glimpse in?
There's varying degrees to which you want to represent your character's inner monologue. You don't really want to bore people with all the random, constant crap that goes through each of our brains on a daily basis, but you do want to represent them realistically.
So how much do you show?
For me, it's all about the nature of that character. If you're writing some crazy-go-nuts person who's distracted by everything around them, popping up bits and pieces of inner monologue can be a great way to express that kind of insanity without just repeating ad nauseum "they twitched crazily." If you've got a stoic hero who doesn't let any of his emotions out, you want to cut a lot of the random chatter. Their inner monologue should be crisp and clean, decisive and short.
If you have a character that constantly talks down to themselves, you're likely to show a lot more of their inner monologue than the driven, no-time-for-doubt problem solver. That doesn't mean that the thoughts of the driven problem solver are less important or illuminating, just that the way that character represents itself isn't with a lot of thinking - it's action.
Too much internal dialogue can slow down a scene where you're trying to describe everyone at the dinner table and the food and the wine, but it can also make that scene more real when the descriptions are accompanied by the thoughts of the person who is experiencing them. Gives the reader another level of immersion.
But the reader is still a reader, not a character, and especially in third person books, you're not asked to get completely inside that character's head. You're viewing them from the outside, learning things about them as you go, and you don't want to give away all your secrets. Too much inner monologue can make things boring, or if the character is particularly nasty, you might make them hate the character so much that they stop reading. And while that might be the point for some novels, in general you probably don't want your main characters to be hate-worthy.
Because this is a blog and not a one-on-one internet counselling session between cyberbrains, I can't tailor make my advice (damn you pre-singularity future!) but my generic advice is to search through your novel and see where a little blurb from your character's thoughts might make it pop, or where you need to pull the camera back. Decide how much of that character is thoughtful and introspective, and how much is derived from action and forward progress. But give us a chance to see inside everyone's heads, if just for a little.
So how much of your character's inner minds do you show? How deep do you let the reader go? How do you mix thought with action and observation?