I had similar issues and I've been trying things for three days. Digital Editions (old version) worked fine on my old Mac, but new mac (Version 10.9.5) and new ADE (Version 4.0) kept crashing whenever I tried to download a book from the B.C. library. Finally figured out a solution. Here's what worked for me.
1. Delete all digital edition files (use "Find" to trash all files and then empty trash).
2. Download Adobe Digital Editions 3.0 (just google this and you'll find the download)
3. Open ADE Version 3.0 (above).
4. Deauthorize the computer (on a mac - CMD, Shift, D). You'll get a prompt asking if you want to deauthorize the computer. Click yes.
5. Authorize computer (under Help window when ADE is open).
6. Go ahead and download book from library (or wherever).
Mine seems to be working fine now.
It is unfortunate that ADE4 kept crashing for you.
It is happening because you got the crash in ADE and when you launch ADE (by double clicking on it). ADE is trying to open the same book and hence getting crashed again and again.
We will address this issue as well in our coming releases.
As a workaround to resolve this, you can try to open any epub file directly by right-click on the file and open-with ADE option.
ADE should be able to open the book correctly.
I will strongly recommend you to use ADE4 as it comes up some good features and support for epub3 format.
I hope it helps.
I tried chiragjuneja's workaround, but ADE kept crashing. It opens the Getting Started Guide (welcome.pub) that came with ADE, but not the two documents downloaded from the Guilford website (one is a sample document, the other is the one purchased). I get the same error message as Lottiotta. I am reluctant to adopt Violet's workaround, but will try it if nothing shows up here in the next few days.
I have ADE4.0 and OS X Yosemite (10.10.1).