The issue of e-book security is a common one, and with your high referral rate from family and friends, I imagine you are more than concerned.
Adobe's PDF is the only system I feel is sophisticated enough to use for producing e-books, and the ultimate solution surely is PDF Merchant and Web Buy.
Until it arrives, a makeshift method would be to use your customer's credit card number as the password protection. A macro program like Quickeys (www.quickeys.com) will simplify the process for you.
There is really no suitable way to protect your e-book from being passed around... though I would doubt that the password would appear on message boards. However, since there are apparently "genuine" serial numbers for pirated software available on the net, anything is possible! I'm just saying I think it's less likely to be a common occurance.
You are correct when you say that I'm not concerned about this issue. One of the reasons is that I expect a certain percentage of copying/fraud, and adjust my prices and expectations to take these into account. In this I've been exceptionally lucky - though, as the saying goes, you make your own luck!
In practice there is quite a bit that we as publishers can do to reduce the loss potential. By setting up the personal dialogue with your prospects and buyers, you can achieve the same effect as shopkeepers are taught in shoplifting seminars... to greet the customer and establish eye contact. This personal approach works because it lifts the veil on the secrecy that lowlifes prefer to operate under. Online marketers achieve the same result by refusing to sell to free email addresses, like Hotmail, where the sender's address is anonymous.
Other protection methods are to tell your buyers that you record IP addresses. Request that they provide a daytime phone number. Get a full postal address.
One of the real advantages in the "how-to" e-book business is the quality of your prospects and buyers. Sure you will get a certain percentage of deadbeats, but generally far fewer than in other, more general areas of publishing. I imagine this factor would be particularly low in your specialised medical information field.
The overall answer is to increase your personal dealings as much as possible. The more anonymous you or your buyers become, the less chance there is of setting up the personal dialogue that is so important to a trustworthy relationship.
PS. As I look over this answer, it becomes apparent that the real solution is not at all technical... it is still the human side of business that provides the real answers. Makes you think, doesn't it?