can't open a zip file (Library Download)

Okay, I am so out of my depth that I don't even know if this is the correct community to post this question. Mea culpa if it isn't.

For the first time, I am using my local library's digital download service to "borrow" HP&Deathly Hallows. (We listened to the first 6 books on cassette, which shows how long we've been listening to Jim Dale.) I've been on the waiting list for about two and finally got the notice yesterday. It said I had 3 days to download it.

So I log in and used Firefox to download it. I get a zip file of HarryPotterandtheDeathlyHollowsUS.odm into my downloads folder. Then I attempt to open it. The default is "Archive Utility" which opens up for about a second. And then nothing. I try it again. I try "The Unarchiver" off the App Store. Try it some more. Still nothing. Nadda. I look in the Finder's "Downloads" folder and there are a whole bunch of HarryPotterandtheDeathlyHallowsUS. They've got interesting suffixes like "-1odm 2.cpgz" and "odm 5.cpgz".

I don't know what any of these mean but how do I get the book into iTunes so I can listen to it? (And keep in mind that I take idiot-proof as a personal challenge.) There's kind of a time limit on how long I can have this book on my computer so I am eager to figure this out.

Any and all help greatly appreciated.

Brian