As someone who has 14 years experience selling
what used to be called "shareware" (and is now called "business as usual on the Web"), I can assure you: If you don't put a price tag on your work, on one else will.
If you want to offer the book for free for a while, do that. Then raise the price later.
Put a price on your work that seems fair to you. Then stick by it.
-David