Well, if you're looking for formal training and certification, check out the Web sites of some of the organizations that certify doulas:www.dona.org
(formerly Doulas of North America)www.icea.org
(International Childbirth Education Association)www.cappa.net
(Childbirth and Postpartum Professional Association)www.alace.org
(Association of Labor Assistants and Childbirth Educators)www.birtharts.org
Each organization definitely has its own "flavor." All the books you're reading are on each organization's required or recommended reading lists. Some do distance education, some require workshop attendance, some require more or less certifying births, etc. If you decide to certify, it may come down to which organizations offer classes in your area ...
That said, anyone can call themselves a doula, whether they have any training or experience or not. The best way to educate yourself is to read, read, read (all the above organizations have reading lists, so that's a great place to start) and talk to other doulas in your area. Find a doula to mentor you, if you can. Attend births -- even just as a silent observer, you'll learn lots. If you know a homebirth midwife, maybe some of her clients will allow you to observe the birth. But becoming a doula involves a lot more than just reading books. You'll more than likely need to pass a breastfeeding exam, a childbirth exam (or attend an accredited class) and maybe do some writing (your thoughts on birth, why you want to be a doula, resource lists for your clients who may have needs that you are unable to meet or whose needs are outside your scope of practice), etc.
Hope this helps!