How Marvel wound up with a "Doctor Strange"
Doctor Strange has spent a good amount of time acting as a supporting character since his 1963 debut in Strange Tales #110, though writers have fed him enough material for Marvel to remix into a movie. Anyone interested in the comics should start at the beginning with Doctor Strange: Marvel Masterworks Vol.1, a collection of comics from Strange creator Steve Ditko and editor Stan Lee that inspires most of the movie's oddities.
Writer Roger Stern was one of the first to dig into the character of Strange. Coupled with artist Mike Mignola (of Hellboy fame), Stern's Doctor Strange and Doctor Doom: Triumph and Torment weave a fascinating tale pairing up two diametrically opposed characters. But somehow Doctor Strange helping Doctor Doom save his mother from hell is one of the more fascinating magic-oriented tales in the entire Marvel Universe. Readers looking for modern takes on the character should check out Doctor Strange: The Way of the Weird, which has beautiful art by Chris Bachalo and writing by Jason Aaron that completely reinvigorates the character. One of the most interesting aspects of the comic is that we learn that using magic changes a mortal body over time. After years of magical ass-kicking, the only food he can eat would kill a normal man (think a gross soup that has eyeballs and tentacles thrown in for good measure) and will eventually kill him.
Where Strange falls on the superhero scale
They don't call him the "Sorcerer Supreme" for nothing. Strange can manipulate time, astral-project, use telekinesis and telepathy, travel through dimensions -- and that's before his spell upgrades and equipped magical objects. At moments in his comics career, he's lost the title and a chunk of his powers. In Mark Waid's Doctor Strange: The Doctor Is Out, he lost the ability to go by the name the Sorcerer Supreme, along with most of his powers and magical gear. The very short run (which may be confusing for new readers) sees him still trying to fight magical menaces as he picks up a teenage girl sidekick who proves to be a little too natural with magic.
Strange also failed big time during the House of M story line, which saw Scarlet Witch depower about 90% of the Earth's mutants. But overall, Strange is considered one of the most powerful humans and mystical characters in the Marvel Universe. He’s battled an Infinity-gauntlet-wearing Thanos, banished the Hulk into space, defeated Death herself, which stopped him from aging, and his longest-running love interest is an extra-dimensional sorceress, so you know he's legit.