The Well-Tempered Tune

Devoted to perfecting the sound of pianos and harpsichords

Information

Web sites

Electronic Tuning Devices
Veritune
Accu-Tuner
TuneLab

Temperaments
About Temperaments - a visual display
Bradley Lehman's spreadsheet
Temperament Offsets
Ed Foote's Web Page

Books:

Piano maintenance and tuning:

Pianos Inside and Out by Mario Igrec. An outstanding reference on everything about pianos. Highly recommended.

Tuning methodology:

On Pitch: The Integration and Equation of Aural and Electronic Tuning Techniques by Rick Baldassin (A small 112 page paperback with a lot of useful insights, but not worth ~$200 on Amazon)
The Craft of Piano Tuning by Daniel Levitan (an excellent book that jumps straight into inharmonicity in chapter 2)
Different Strokes: Hammer Techniques for Piano Technicians by Ken Burton. (a great compendium of hammer methods)
The Theory and Practice of Piano Tuning by Brian Capelton.

Temperaments:

Tuning: Containing the Perfection of Eighteenth-Century Temperament, the Lost Art of Nineteenth-Century Temperament and the Science of Equal Temperament by Owen H. Jorgensen (the ultimate source on historical temperaments)
Temperament: How Music Became a Battleground for the Great Minds of Western Civilization by Stuart Isacoff (an excellent history of temperament, but lacking in a clear explanation of the math)
The Seventh Dragon: The Riddle of Equal Temperament by Anita Sullivan

Mathematics of Sound:

Musimathics by Gareth Loy (an excellent text, but amazingly there is nothing on stretch and inharmonicity)

Interesting papers in the professional literature:

Tuning Preferences for Piano Unison Groups by Roger Kirk
Coupled Piano Strings by Gabriel Weinriech
The Inharmonicity of Plain Wire Piano Strings by Robert Young
Influence of Inharmonicity on the Tuning of a Piano - Measurements and Mathematical Simulation by Jean Lattard
String Inharmonicity and Piano Tuning by R. Rasch and V. Heetvelt