multiple devices
If I purchase a book on my Mac, can I see it on other Macs, IPads, and IPhones?
If you mean an ibook then yes, as long as it remains in your country's store then you can redownload it via the Purchased tab in the ibookstore in the iBooks app on your devices, and the Purchased link under Quicklinks on the right-hand side of the iTunes store homepage on your Macs iTunes. Or you can manually sync/copy it over.
If you mean an audiobook then they are one-time only downloads from the store, you will need to sync to any iOS device that you want it on, and copy it over to your other Macs.