If you want that while composing a message, pressing the F9 key may address your concern.

Thanks david, I was specifically hoping for seeing the address book and the mailboxes side-by-side, so not while composing. I've realized that this is achieved by opening the address book (in a tab), and then dragging the tab onto the desktop to make a new window (which also creates a 2nd tab with mailbox view, along with the dragged address book). That feels a little hokey, and takes 2 steps for what might be a 1-step process. But it does get the job done. I would still welcome a definitive answer that there isn't a smoother way to get a separate address book window.

Well, I'm impressed with your solution. :) My inference is that the addressbook will have expanded capability in future, but for now it's what it is. Thanks for sharing.