I knew Mark when he was at the School of Public Policy at UCLA, and much enjoyed his company; I highly recommend the substantive and gracious obituary here at Reason by Jacob Sullum, who worked in the same field as Mark did. An excerpt:
Back in 1989, Mark Kleiman published a book, Marijuana: Costs of Abuse, Costs of Control, that exemplified his calm, methodical, just-the-facts approach to drug policy. Kleiman argued that federal efforts to curtail cannabis consumption were ineffective and diverted resources from programs that had a better public safety payoff. Three years later, in Against Excess: Drug Policy for Results, he came out in favor of legalizing marijuana, arguing that the costs of prohibition outweighed its benefits. At a time when three-quarters of Americans still supported marijuana prohibition, Kleiman’s position was striking, especially coming from a widely quoted and consulted academic who had the ear of policy makers….
I did not always agree with Kleiman’s conclusions [such as Kleiman’s support for continued criminalization of drugs other than marijuana and psychedelics], but I admired his method, which acknowledged subtleties and uncertainties, anticipated counterarguments, and insisted on empirical support for claims that were frequently asserted as articles of faith.
“Eventually we must learn to discuss our drug policies without raising our voices,” Kleiman wrote in Against Excess. “A drug-crazed drug warrior can be as great a public menace as a drug-crazed addict.” He never lost sight of the burdens imposed by coercive drug policies, even when he supported them.