My partner Scott D. Roberts and I do book coaching as part of our services at 3L Publishing (www.3LPublishing.com). An interesting weakness we often see in writers is the inability or skill to pace their books.
Here are three tips about pacing:
#1 -- Using Too Much Exposition. A big no-no is too much exposition in the narrative that bogs down the writing. You have to keep your eye on what you're trying to accomplish on each page. Each scene, each moment needs to move the story forward or have a purpose in the story. We often see new writers who mistake a great description as building a colorful setting. Your description should only feature that which does the following:
#2. Helps define the scene so the reader understands the place. For example, if you have a scene in an office you need just enough exposition to convey the nature of the office and that's all. This gives the reader an idea of it so they can picture it in their minds. For example, a lawyer's office might have a "neatly arranged law library that any librarian would drool over." That's enough. The reader got it. We don't need to spend two or three more paragraphs describing that law library, especially if the scene isn't even about libraries and has no relevance to your story.
#3. Repeating what you just described in the dialog. If you told the reader your hero entered a bar and ordered a drink, don't have the dialog essentially repeat what you described. The reader got it the first time. For example, don't have the hero enter the bar and say, "Hey, I just walked in the bar," to his friend on the phone. It will bore the reader and bog down your narrative in redundant storytelling.
#4. Dialog that is more of a lecture than a conversation. Listen to how people talk. Most people don't go on and on and on. Unless you intend to make that a character trait specific to one person, it bogs down the work, and makes your dialog weak. People talk generally back and forth and don't use formal vocabulary unless it's a period piece. So make sure you keep the language current to slang and lingo, too.