You might be able to - what does the book's description page in the store say that it's compatible with ?
If you don't already have the iBooks app on your first gen iPad then, and if the app isn't already in it, download the current version on your computer's iTunes so that the app is in your account's purchase history. Then on your first gen iPad go to the Purchased tab in the App Store app on it and you should be able to download an older compatible version of it.
If the book is compatible with that version of the iBooks app it should show in the Purchased tab in the iBookstore in the app for redownloading on it