You know that you can lookup the url of a ressource by opening the 3-dots menu behind e.g. a calendar and choosing properties? That’s not exactly what you asking for but a workaround. I would assume that the base url is usually the part in front of the user name.
I understand the single address book per account issue and agree with keeping it as is. However, it would be nice to have the capability of copying an account so if there are numerous books it’s not necessary to enter there login information over and over. This may be an Android limitation, but the ability to change the name of the account would also be nice.