The classic Martini is probably one of the most well known cocktails. As with many cocktails there are a number of different stories regarding it's history but both Gin Foundry and Imbibe by David Wondrich provide the most accurate backgrounds.
The Martini's recipe is often debated but the one listed is a good base to start with. Want it dry? Use less vermouth. Want it Perfect? Add sweet vermouth. Like it dirty? Add some olive brine. There are endless options.