I'm just getting into that idea now. I am creating a free download on Natural Solutions for Fibromyalgia that I'll be giving away on my site.
A friend also suggested giving something away to those who subscribe to my newsletter as an incentive to get people to sign up. I went to a site called
http://www.free-ebooks.net/ to see what they had to offer.
I found 3 that would suit the spirit of my website so I emailed each author to get permission to use their ebook. The first to respond, less than 24 hours later was
Sheila Dicks who wrote an ebook How to Look 10 Years Younger. What I liked about it was that she included lots of natural stuff and her website and blog seemed appropriate for me to be recommending.
So this is one option. Rather than write it yourself, you can find free content to give away.
The book I've been using to improve my marketing skills, the Renegade Network Marketer stresses giving away lots of free content in the form of newsletters, downloads, ebooks, etc to build customer loyalty.