R. B. Greaves, an R&B singer whose 1969 hit "Take a Letter, Maria" reached No. 2 on the Billboard pop chart, died on Thursday in Los Angeles. He was 68. Mr. Greaves was a nephew of the gospel and soul singer Sam Cooke, who was shot and killed by a Los Angeles motel manager in 1964. Mr. Greaves’s 1970 version of Burt Bacharach and Hal David’s “Always Something There to Remind Me" reached No. 27 on the Billboard chart.