Basic reading:

"Eyewitness to Gettysburg", by Charles C. Coffin (Burd Street Press)

"Gettysburg", by Stephen W. Sears (Mariner Books)

"Gettysburg: A Battlefield Atlas", by Craig L. Symonds (Nautical & Aviation Publishing)

"Gettysburg: A Battlefield Guide", by Mark Grimsley and Brooks D. Simpson (University of Nebraska Press)

"Gettysburg: A Testing of Courage", by Noah A. Trudeau (HarperCollins)

"Gettysburg: By the Numbers", by Charles Teague (Adams County Historical Society)

"Gettysburg: 1863 and Today", by Charles R. Nitchkey (Exposition Press)

"High Tide at Gettysburg: The Campaign in Gettysburg", by Glenn Tucker (Morningside Press)

"The Battle of Gettysburg", by the Comte de Paris (Butternut and Blue)

"The Battle of Gettysburg: A Guided Tour", by Edward Stackpole and Wilbur Nye (Stackpole Books)

"The Battle of Gettysburg: The Eyewitness Account of Col. Frank A. Haskell", by Frank A. Haskell (Chapman Billies)

"The Gettysburg Campaign", by Edwin B. Coddington (Morningside Press)

"The Gettysburg Campaign: June and July, 1863", by Albert A. Noh (Gallery Books)

"The History Buff's Guide to Gettysburg", by Thomas R. Flagel and Ken Allers, Jr. (Cumberland House) 

"The Maps of Gettysburg: An Atlas of the Gettysburg Campaign, June 3-July 13, 1863", by Bradley M. Gottfried (Savas Beatie LLC)

"The U.S. Army War College Guide to the Battle of Gettysburg", by Jay Luvaas and Harold W. Nelson (HarperCollins)

"Witness to Gettysburg", by Richard Wheeler (Harper & Row)

July 1, 1863:

"Crisis at the Crossroads: The First Day at Gettysburg", by Warren W. Hassler, Jr. (University of Alabama Press)

"Gettysburg: The First Day", by Harry W. Pfanz (Chapel Hill)

"In the Bloody Railroad Cut at Gettysburg", Lance J. Hergedon and William J. K. Beaudot (Morningside Press)

"Morning at Willoughby Run: July 1, 1863", by Richard S. Shue (Thomas Publications)

"The First Day at Gettysburg:  A Walking Tour", by James E. Thomas (Thomas Publications) 

 July 2, 1863:

"Culp's Hill at Gettysburg: The Mountain Trembled", by John M. Archer (Thomas Publications)

"Devil's Den: A History and Guide", by Garry E. Adelman and Timothy H. Smith (Thomas Publications)

"East Cemetery Hill at Gettysburg: The Hour Was One of Horror", by John M. Archer (Thomas Publications) 

"Gettysburg: Culp's Hill and Cemetery Hill", by Harry W. Pfanz (Chapel Hill)

"Gettysburg: The Second Day", by Harry W. Pfanz (Chapel Hill)

"Little Round Top: A Detailed Tour Guide", by Garry E. Adelman (Thomas Publications)

"The Attack and Defense of Little Round Top", by Oliver W. Norton (Morningside Press)

"The Wheatfield at Gettysburg: A Walking Tour", by Jay Jorgensen (Thomas Publications)

July 3, 1863: 

"Gettysburg: Day Three", by Jeffry D. Wert (Simon & Schuster)

"Nothing But Glory: Pickett's Division at Gettysburg", by Kathy Georg Harrison and John W. Busey (Thomas Publications)

"The Struggle for the Bliss Farm at Gettysburg", by Elwood Christ (Butternut and Blue)

Artillery:

"Silent Sentinels:  A Reference Guide to the Artillery at Gettysburg", by George W. Newton (Savas Beatie LLC)

"The Guns at Gettysburg", by Fairfax Downey (Olde Soldier Books)

"Three Days at Gettysburg: July 1, 2 and 3, 1863", by Henry J. Hunt (Outbooks)

Cavalry:

"Encounter at Hanover: Prelude to Gettysburg", by the Hanover Chamber of Commerce

"Gettysburg's Forgotten Cavalry Actions", by Eric J. Wittenberg (Thomas Publications)

"Plenty of Blame to Go Around: Jeb Stuart's Controversial Ride to Gettysburg", by Eric J. Wittenberg and J. David Petruzzi (Savas Beatie LLC)

"Protecting the Flank: The Battles for Brinkerhoff's Ridge and East Cavalry Field, Battle of Gettysburg", by Eric J. Wittenberg (Ironclad Publishing)

"Saber & Scapegoat: J.E.B. Stuart and the Gettysburg Controversy", by Mark Nesbitt (Stackpole Books)

"The Cavalry at Gettysburg: A Tactical Study of Mounted Operations During the Civil War's Pivotal Campaign, 9 June - 14 July 1863", by Edward Longacre (University of Nebraska Press)

"The Devil's to Pay: General John Buford, USA", by Michael Phipps and John S. Peterson (Farnsworth Military Impressions) 

Civilians:

"At Gettysburg, Or What a Girl Saw and Heard of the Battle", by Mrs. Tillie Pierce Alleman (Stan Clark Military Books)

"Days of Darkness: The Gettysburg Civilians", by William G. Williams (White Mane Books)

"Women at Gettysburg: 1863", by E.F. Conklin (Thomas Publications)

Human Interest Stories:

"My Enemy, My Brother: Men and Days of Gettysburg", by Joseph E. Persico (Collier Books)

"On the Bloodstained Field: 130 Human Interest Stories of the Campaign and Battle of Gettysburg", by Gregory A. Coco (Thomas Publications)

"On the Bloodstained Field II: 132 More Human Interest Stories of the Campaign and Battle of Gettysburg", by Gregory A. Coco (Thomas Publications)

"War Stories: 150 Little Known Human Interest Stories of the Campaign and Battle of Gettysburg", by Gregory A. Coco (Thomas Publications)

Lee's Retreat:

"One Continuous Fight: The Retreat from Gettysburg and the Pursuit of Lee's Army of Northern Virginia, July 4-14, 1863", by Eric J. Wittenberg, J. David Petruzzi, and Michael F. Nugent (Savas Beatie LLC)       (NEW)

"Retreat from Gettysburg: Lee, Logistics, and the Pennsylvania Campaign", by Kent Masterson Brown (Chapel Hill)

 

Monuments:

"Gettysburg: The Complete Pictorial of Battlefield Monuments", by D. Scott Hartwig and Ann Marie Hartwig (Thomas Publications)

"The Location of the Monuments, Markers, and Tablets on Gettysburg Battlefield", by Kathy Georg Harrison (Thomas Publications)

Photography:

"Early Photography at Gettysburg", by William A. Frassanito (Thomas Publications)

"Gettysburg: A Journey in Time", by William A. Frassanito (Scribners)

Strategy:

"Lee and Longstreet at Gettysburg", by Glenn Tucker (Bobbs-Merrill) 

"Lee's Real Plan at Gettysburg", by Troy D. Harman (Stackpole Books)

Other Topics:

"Beyond the Gatehouse: Gettysburg's Evergreen Cemetery", by Brian A. Kennell (Evergreen Cemetery Association)

"East of Gettysburg: A Gray Shadow Crosses York County, PA", by James McClure (York Daily Record / York County Heritage Trust)         

"The Gettysburg Nobody Knows", by Gabor S. Boritt (Oxford University Press)

"Wasted Valor: The Confederate Dead at Gettysburg", by Gregory A. Coco (Thomas Publications)