The answer comes (verbatim) courtesy of John McComb in the Logos Newsgroups:

Suppose you want to put all your bibles (for instance) into one button. For the sake of this example we'll call the drop down toolbar the 'bibles' toolbar and the button you create to access it the 'all bibles' button.
Just follow these steps:

The Bible Bar Script is a combination of a script and toolbar. The toolbar provides the buttons and a script provides the drop down menus. These are proper drop down menus which appear as the cursor is moved over the buttons. Each button represents a collection. There is also an option to open the entire collection.

THIS SCRIPT REQUIRES THE CUSTOM STARTUP SCRIPT!

THIS SCRIPT IS REPLACED BY THE COLLECTION TOOLBAR SCRIPT AND SHOULD NOT BE RUN AT THE SAME TIME AS THAT SCRIPT!

Installation instructions are included in download.

Libronix is nice, but I really dislike the way it automatically positions my windows. I know I can turn that off, but it turns out to be even worse for me. So I leave that function on. And then I reposition my books using one of these two toolbars.
IMHO they are a must have: