Good resources shared by others.
One aspect I've found enlightening is to play around with SS benefit estimate calculators and see how occasional part-time work can really impact future benefits in place of a bunch of zeros. As the first 'bend point' is at $926 ave monthly wages (about $11,000 annually) and results in 90% benefits, for early retirees* earning this amount per year can result in a substantial shift, which may or may not be important to you.
*yes, call the IRP on me for suggesting some 'work' during ER. :-P