eBooks come in different formats, some are only available as PDFs, others might be available in ePub format. You can open ePubs on your Apple and Android devices but not on Kindle. Axis 360 has it's own app that you can download to mobile devices, including the Fire.
If you are a looking for a specific title
If you are a college student in Massachusetts or work in Massachusetts, you are eligible for a Boston Public Library eCard! You can sign up online and check out eResources without ever having to go anywhere.