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

Okinawa: the last Battle:  by Roy E. Appleman, James M. Burns, Russell A. Gugeler, and John Stevens, US Army

Ryukyus: The U.S. Army Campaigns of World War II

Hyperwar World War II : Pacific Theaterof Operations

Operation Iceberg: The Assault on Okinawa : The Last Battle of World War II (Part 2) April - June 1945, Antill, P. (23 May 2003)

http://www.96th-infantry-division.com/memorials/okinawa/okinawa.htm


USMC

Okinawa: Victory in the Pacific: by Major Chas. S. Nichols, Jr., USMC, Henry I. Shaw, Jr. USMC History Book

The Final Campaign: Marines in the Victory on Okinawa by Colonel Joseph H. Alexander, USMC (Ret)

History of U.S. Marine Corps Operations in World War II Volume IV: Western Pacific Operations: By George W. Garand, Truman R. Strobridge

History of U.S. Marine Corps Operations in World War II Volume V: Victory and Occupation: By Benis M. Frank and Henry I. Saw, Jr.

Sixth Marine Div : A Brief History of the Battle of Okinawa



US Navy

Davy Jones' War : Naval operations (independent of amphibious operations) Convoys, carrier raids, submarine operations, etc.

HISTORY OF U.S. NAVAL OPERATIONS IN WORLD WAR II: by Samuel Eliot Morison Vol. XIV: Victory in the Pacific

Building the Navy's Bases in World War II: Chapter 30-- Okinawa

AMPHIBIOUS OPERATIONS: Capture of Okinawa (Ryukyus Operation) 27 March to 21 June 1945

The Story of Fleet Logistics Afloat in the Pacific During World War II, Beans, Bullets, and Black Oil: by Rear Adm. Worrall Reed Carter
USN (Retired)

U.S. Navy at War 1941 - 1945 Official Reports to The Secretary of the Navy: by Fleet Admiral Ernest J. King, U.S. Navy

OPERATIONS in the PACIFIC OCEAN AREAS: During the Month of AUGUST 1945

Losses of United States Naval Vessels from all causes: 7 December 1941-1 October 1945

STATUS OF MAJOR COMBATANT SHIPS OF JAPANESE NAVY

U.S.S Emmons Photo gallery
Site 1
Site 2



USAAF

The Army Air Forces in WWII: Vol. V: The Pacific: MATTERHORN to Nagasaki, June 1944 to August 1945

The Campaigns of the Pacific War: Chapter 14: The Okinawa Campaign

The Army Air Forces in WWII: Vol. V: The Pacific: Chapter 19 Iwo Jima and Okinawa

Airfield locations on Okinawa

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.

http://www.Footnote.com/WWII/  This site is absolutely amazing: Over 80,000 WWII pictures, 47 million documents. Anything you'd like to see.

http://www.ibiblio.org/pha/USA-in-WWII/Set-2-of-7/Disk-3/Okinawa/

United States Strategic Bombing Survey: Interrogations of Japanese Officials

http://www.pacificwrecks.com/provinces/japan_okinawa.html
Photo Gallery
Reference Links Page
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List of Names
Engraved on the
Peace Memorial Wall