SAMM SINCLAIR BAKER, 87, whose 30-odd self-improvement books counseled readers on how to help themselves on anything from svelte figures to good sex and making money by writing books, died March 5 at United Hospital in Port Chester, N.Y. He had suffered a stroke. Mr. Baker left his career as a copywriter and agency executive in 1963 to write books and continued working as a business consultant. He was a co-author with several diet and fitness experts on their best-selling advice books. Some of these included the Doctor's Quick Weight Loss Diet, with Dr. Irwin Stillman, and the Complete Scarsdale Medical Diet, with Dr. Herman Tarnower. He published three mystery novels, three gardening books and works on creative thinking.