There is several webpages out there that are a wealth of knowledge about the Battle of Okianwa. This page is just for that. Over the past year, while researching for this site. I've stumbled across several great links along with a couple online books. Below is a list of a some of my personal favorites.
Good luck searching!
Online Books & References:
US Army
USMC
The Final Campaign: Marines in the Victory on Okinawa by Colonel Joseph H. Alexander, USMC (Ret)
US Navy
Davy Jones' War : Naval operations (independent of amphibious operations) Convoys, carrier raids, submarine operations, etc.
USN (Retired)
U.S.S Emmons Photo gallery
USAAF
Misc.
Combat Arms Research Library: This is by far one of the most interesting research tools I've found yet. On the search bar type Okinawa and be prepared to not go anywhere for several hours. All data compiled on this site is scanned documents, as you will see is Declassified.
My personal favorites are: G-2 Summary Report, Information Bulletin, Okinawa Report, and Unit Orders and Reports
Lastly, depending on your internet speed, these might take a while to download. It is absolutely worth the wait. These reports are crammed with everything from maps, to personal accounts, to ammunition inventory.
Leavenworth Papers: by Dr. Thomas M. Huber: Everything you could ever want to know about the IJA.