I just got super lucky. I launched it on Kickstarter and did a post on /r/arduino the day of the launch that just kind of blew up over there, and that got me to my 10k funding goal on the first day. Then someone stole the video and crossposted on LinkedIn, and that REALLY blew up. The post got like 1M views or something. They didn't share a link to the Kickstarter or anything, so I was getting a lot of traffic but it looked like it was all showing up as direct/search traffic, because people had to go to google and actively seek out the book. Halfway through the campaign I brought on an ad agency that just does Kickstarter campaigns, and I had a pretty good experience with them as well. The campaign ended at just under $250k.
Since then I've continued to take pre-orders and have just been doing facebook ads. I expect that when people start getting their books, that will help drive a lot more traffic, and then I hopefully won't have to be so reliant upon facebook ads.