I went with Lulu, a POD (Print on Demand) service for both books because there is no initial investment required (none, only your time) which allows you to make changes and improvements to your book on the fly. I can't tell you how important that is!
Once you've got your book written, edited, formatted and released to the public via Lulu, Lulu (for a fee) will get you into the big online retailers like Amazon.com, Barnes and Noble and a bunch of small ones. I've seen Zero to Superhero in online stores based in Norway (or was in Finland?) and niche online vendors elsewhere in the world.
And you can have it both ways. You can have a conventional run done as well from Lulu (pricier though). It's a win/win.
I probably sound like a pitchman for Lulu, but it's a great service. I can't understand why someone would take a big upfront risk and do a traditional launch without testing the market first (and a POD soft launch is perfect for that). You'd have to be pretty darn sure that you've got pre-built demand for the book, and that your marketing plan is a solid multi-channel one (you'll need that anyway).
Good luck to you.