FWIW: very, very weird - I am able to get 6.11 to select multiple books with an unmodified left mouse button down drag, see attachment. But it's a little bit tricky.
I got the following by: a) selecting book 4, b) I then moved the mouse pointer down to book 7 (book 4 remains selected), c) and then, with left button down, I moved the pointer downwards.
If I don't do b) and c) as a single 'flowing' action, book 7 is 'grabbed' and I can drop onto the desktop etc.
But, when I tried the same thing on version 6.10 portable, I couldn't induce the same behaviour. But that install is very close to factory settings, no optional plugins, no customised tweaks. Some toolbar changes.
Look and Feel->Allow using drag and drop to merge books is unchecked in my 6.11 desktop install and my 6.10 portable install.
BR