Brainstorm a list of "special people." Don't edit yourself. Sit down and write down any and every person that you can think of who helped and/or inspired you during your book writing process. If you wrote a book that required a lot (or even a little) research, don't forget those people that helped you with that. The key at this point is not to edit. From the bagel delivery guy to your mom and dad.
Make "yes," "no" and "maybe" piles. Go through your list and use your first instinct. Look at the name. If your immediate response is "wow, did they help me," that person goes in the "yes" pile. If your reaction is, "why is that person's name here," they go in the "no" pile. If you sit in front of the name for more than fifteen seconds and seriously cannot go one way or the other, put them in the "maybe" pile.
Get rid of the "no" and "maybe" piles. It's as simple as that. If you had to think about the person's contribution to your process for more than fifteen seconds, they may have helped in some way, but their name does not belong on the acknowledgments page.
Narrow the list down to your top ten. There's nothing wrong with acknowledging someone on your acknowledgment page that did something as trivial as delivered your lunch every day, if it was something that stuck in your mind and really meant something to you. The acknowledgment page should be personal and reflect your process. Think about your "yes" pile and start filling in the top ten people that really stick out the most. Some will jump right up to number one. Others may creep up on you.
Throw out five through ten. Again, they may have helped you or inspired you to a certain degree, but if your initial reaction was not "They must be thanked," then send them a fruit basket instead.
Write out your acknowledgment in several short paragraphs. If there were several people who helped with research, thank them in one paragraph, while thanking friends and family that gave you moral support in a following paragraph. Keep it short, but sweet.