Too long? Too short? Just right?
The answer of what makes a good length for an ebook really depends on the "3 P's" of ebook strategy.
Side note: when I say "page" in the discussion below, I am referring to a single-spaced, 12-pt type, 1-inch page margin, 500 word page in your word processor.
#1 P = Purpose
What's your purpose in writing your ebook? If it's to build up a list by giving it away for free, you can go with a "report-length" ebook of 5-15 pages. Though you may call it a "special report" it's still an ebook.
If your purpose is to create a "lead generation" ebook where people pay an entry-level price for the ebook, then your ebook would typically be around 20-40 pages.
If your purpose is to create an ebook that's a major profit center for your business and sells for $20, $30, $50 or $100, then your ebook typically needs to be around 50-100 pages.
#2 P = Promise
What's the promise of the ebook you've written? How much content do you need to fulfill that promise and a little bit more (over-deliver)? I think the promise is as important, if not more important, than the actual content of the ebook anymore. If you have a great promise and fulfill it, I don't think people care nearly as much about how long the ebook is (within reason).
The key however, is to fully deliver on the promise AND make people feel good about having made the purchase. If your ebook is too short, people feel gypped or short-changed. If it's too long, they want you to just "get to the point" already. So promise and length of ebook are as much about feeling as anything else. Put yourself in the shoes of your typical audience member and ask yourself "Based on the promise and how I fulfill it, what feels like a good length here?"
#3 P = Price
Price is tricky. One man's deal is another man's rip-off. A 12-page ebook for $100? I don't think so - at least not for a general audience! A 100-page ebook for $100? Ok. I might buy that based on the promise and a few other factors. $10 for a 1,000 page ebook? Probably not - I'm sure it would wander aimlessly.
Bottom line on price and length of your ebook: based on your promise and your purpose, price your ebook about where others have theirs priced and make your ebook long enough to make people feel good, but short enough to avoid frustrating them.
You can't really go wrong if you get very clear on the purpose of your ebook, make a compelling promise, and charge a fair price. If you focus on making your ebook and the purchase of it a great experience for your customers, then the length of your ebook will just take care of itself (literally).