Leiter makes his first appearance in the very first Bond book, Casino Royale. In the books he and Bond are friends with a history of working together. In Live and Let Die, the second Bond novel, Felix is fed to a shark and loses an arm and a leg. He is subsequently invalidated out of the CIA, but continues to appear throughout the series.
In the movies Bond and Leiter meet for the first time in Dr. No, the first Bond movie. The film version of Live and Let Die does not contain the sequence with the shark, and Leiter survives intact to help Bond again - appearing in a total of eight movies, played by seven different actors, widely varying in physical characteristics and even race. Leiter's final film appearance was in Licence to Kill. Leiter is married, but his wife is killed and Leiter himself severely mutilated (having finally run into that shark), causing Bond to set out on a mission of revenge which forms the movie's central plot. Interestingly, the only two Bond movies in which Leiter is played by the same actor (David Hedison) are Live and Let Die and Licence to Kill...
In the 1954 American television adaptation of Casino Royale, which featured Barry Nelson as CIA agent Jimmy Bond, his opposite number became British secret agent Clarence Leiter.