Ok, I was curious so I took a crack at trying to find Tim's easier way. I was partially successful.
If you open the Store in iBooks and go to the Purchases tab at the bottom, you will see a list of books you bought. By default they are in the order purchased. If not, you can change this with the Sort By button at the top right.
The purchased date is listed, but unfortunately it's usually obscured by the Download/Downloaded button. If you are in landscape view, all you can see is the word Purchased..., but if you turn it to portrait view you can see the full date for books that you can Download, or just the month and day for books that are already Downloaded.
So, at the minimum you can get the order of the purchases and the month/day. This might be enough, or not, depending on your needs.
The other method has to be done on iTunes, but works much better.
First, you have to have your books synced to iTunes on the computer for this to work. Just look under the Books category under the Library list in the left column and see if your books are there.
If not, you'll need to connect your iPad to iTunes, select the iPad from the Devices list, go to the Books tab at the top and set up syncing, then right click on the iPad under devices list and choose transfer purchases.
This will copy everything you've ever bought through one of Apple's store apps to iTunes. It might take a while if there is a lot of stuff to copy over. (if that does not make sense, ask and I'll expand on it)
Once you do that to go the Books list under the Library heading in the left column. Right click in the title bar (top of the list where it says things like Name, Author, etc.) and choose Purchase date. This will add a purchase date column to your listing.
You can change the order of the lists columns by dragging them with the mouse.
Not every book in my list has a purchase date, but all the ones I bought through the iBooks Store do.