iBook

Not all ibooks are compatible with iPhones, so you may not be able to download them on (or sync them to) your phone - an ibook's requirements should be listed on its description page in the store.

For ibooks, then as long as they are still in the store then what you can redownload will show in the Purchased tab in the iBooks app on your devices for downloading on them.

For books from other sources you will have to sync them to your other devices via your computer's iTunes, they won't show in the cloud for redownloading on them. If they are only one of the device and not on your computer then you should be able to copy them over to your computer by connecting the device to your computer's iTunes and doing File > Devices > Transfer Purchases (that should copy epubs and/or PDFs in the device's iBooks app, not just actual ibooks) - if using a Mac with Mavericks or Yosemite on it then do File > Move Books From iTunes in its iBooks app to move them over to that. When you connect your other devices to your iTunes they should then show on their Books tab for syncing to them.

If you want to keep your bookmarks in your places, and the collections that you put the books into, in-line between your devices, then enable Settings > iBooks > Sync Bookmarks and Notes and Sync Collections 'on' on each device.