There are many excellent sources of information out there in print and online.  This is a listing of some of the more useful items I have encountered:


The excellent OOB work of Dr. Leo Niehorster, some of which can be found online at niehorster.orbat.com
The Armchair General at  rkkaww2.armchairgeneral.com  (great Russian stuff)
The ComsimWorld Forum (more than 18000 posts on PB/PL and growing)
The Imaginative Strategist website by Ward McBurney (a true gentleman)
The invaluable input and critique of Byron Henderson

                             General Reference Books

British and American Tanks of World War II by Chamberlain and Ellis

World War II Order Of Battle by Shelby L. Stanton (very thorough listing of all U.S. ground forces from battalion through division)

Czechoslovak Armored Fighting Vehicles 1918-1948 by Kliment & Francev
Encyclopedia Of German Tanks of World War Two by Chamberlain and Doyle
Panzer Trupen : The Complete Guide to the Creation & Combat Employment of Germany's Tank Force 1933-1942  by Thomas Lentz
Panzer Trupen : The Complete Guide to the Creation & Combat Employment of Germany's Tank Force 1943-1945  by Thomas Lentz
Jane's World War II Tanks and Fighting Vehicles The Complete Guide by Leland Ness
Russian tanks of World War II by Bean and Fowler
Tanks of World War 2 by Chris Ellis  (lots of nice color illustrations)
Red Army Handbook by Zaloga and Ness - excellent resource for Russian organizations

British and CommonWealth Armies 1939-43 (Helion Order of Battle 1) by Mark Bevis  -  Excellent OOB info

British and CommonWealth Armies 1944-45 (Helion Order of Battle 2) by Mark Bevis  -  Excellent OOB info

British and CommonWealth Armies 1939-45 Supplement 1 (Helion Order of Battle 3) by Mark Bevis  -  Excellent OOB info

British and CommonWealth Armies 1939-45 Supplement 2 (Helion Order of Battle 4) by Mark Bevis  -  Excellent OOB info

The German Order of Battle - Infantry in World War II by George F. Nafziger  - an essential reference, much detail

The German Order of Battle - Panzer and Artillery in World War II by George F. Nafziger  - an essential reference, much detail

The German Order of Battle - Waffen SS and Other Units in World War II by George F. Nafziger  - an essential reference, much detail

Rumanian Order of Battle by George F. Nafziger

Bulgarian Order of Battle by George F. Nafziger

                              North Africa

Tank Combat in North Africa (The Opening Rounds) by Thomas Jentz

Operation Compass 1940 - Osprey Campaign Series 73 by Jon Latimer

The Desert War by George Forty

Tobruk 1941 - Rommel's Opening Move - Osprey Campaign Series 080 by Jon Latimer

Rommel's Africa Korps - Tobruk to El Alamein - Osprey Battle Orders 020 by Pier Paolo Battistelli

Afrika Korps 1941-43+ - Osprey Elite Series 034 by Gordon Williamson

Panzer Armee Afrika - Tripoli to Tunis - Tanks Illustrated 028 by Peter Gudgin

Armor of the Deutsches Afrikakorps - Concord Publications 7021 by Tom Cockle

The German Army 1939-45 (2) North Africa & Balkans - Osprey Men-at-Arms 316 by Nigel Thomas

Desert Rats. British 8th Army in North Africa 1941-43 - Osprey Battle Orders 028 by Tim Moreman

The Italian Army 1940-45 (2) Africa 1940-43  - Osprey Men-at Arms 349 by Philip S. Jowett

The British Army 1939-45 (2) Middle East & Mediterranean  -  Osprey Men-at-Arms 368 by Martin J. Brayley

7th Armored Division - The 'Desert Rats' by George Forty

The British 7th Armored Division 1940-45 - Osprey Vanguard 1 by John Sanders

The Australian Army in WWII -Osprey Elite Series 153 by Mark Johnston

British Tanks in N. Africa 1940-42  - Osprey Vanguard 23 by Bryan Perrett

Matilda Infantry Tank 1938-45 - Osprey New Vanguard 8 by David Fletcher

Crusader Cruiser 1939-45 - Osprey New Vanguard by David Fletcher

The 25-pounder Field Gun 1939-72 - Osprey New Vanguard 48 by Chris Henry 

                          Battle of the Bulge

The Ardennes: Battle of the Bulge by Hugh M. Cole  U.S. Army in WWII, European Theater of Operations

The Battle of the Bulge Time-Life Books World War II by William K. Goolrick and Ogden Tanner

Ardennes 1944 - Osprey Campaign 005 by James R. Arnold

Battle Of Ardennes - Northen Shoulder - Osprey Campaign 115 by Steven J Zaloga

Battle of the Ardennes 1944 (2) Bastogne - Osprey Campaign 145 by Steven J. Zaloga