Three proposals sent, two books purchased and the third is actively in talks of being picked up.
I think if you write a good, thorough, and cohesive book proposal... that is half your battle.
I totally agree with this. Based on my experience/background, book subject matter, and where I wanted to go with it, I decided I really wanted an agent. To get one, I knew I had to write a really good proposal, and I spent a couple of weeks putting one together; it ended up being around 50 pages long, and I followed a couple of templates I found on publisher websites that seemed to be pretty standard. I only wrote three sample chapters, following advice I had seen on many publishing sites and forums, and I'm really glad I didn't spend more time/effort than that. I pitched roughly 50 agents over the course of two weeks (all via email), and got requests to see the proposal from five. I linked up with an agent shortly thereafter, and she worked with me extensively to refine my existing proposal before she started pitching it to publishers. This was an important step, and a major reason I would encourage non-fiction authors to get an agent first. Mine knows the editors' likes and dislikes at several different houses, so she was able to help me craft the proposal in a way that was more appealing to them - something I wouldn't have been able to do on my own.
Anyway, you have to know your subject matter cold, and be prepared to answer many questions on the spot from a potential agent, and later down the road, a potential editor. Who is your target market? Here's a BIGGIE - what is your platform? Why would people want to read your book, and what resources can you leverage to help a publisher make money off your book? All that needs to go into the proposal, and be at the tip of your tongue if necessary.