Oh boy, it's a huge subject. I'd actually start with the wiki entries on nuclear warfare & countervalue vs. counterforce targeting. The National Security Archive has some very good briefing books on the evolution of the SIOP which deal with the flexibility policymakers started to want over time as well as debates about launch on warning (where you fire once you believe you're under attack) vs. launch under attack (where you ride out at least some of the first wave).

Alex Wellerstein's blog is a good one and he links to other quality sources as well (though note some are decidedly abolitionist when it comes to nuclear weapons).