Give the user a reason to want to complete the form in the first place. Like others have said don't be afraid to give your visitors a taster of what's to come in the form of a partial book or a screenshot of your Whitepaper. You might also want to think about other techniques that are commonly deployed in online stores to get people to purchase. So think about things such as allowing users to rate or comment on your content so that other visitors can get unbiased opinions before they download. You should also use things like progressive profiling so that you are only ever asking for minimal information at any one time, this reducing some uncertainty people have about giving away their personal details.