At large on Pingelap in the remote Pacific, Oliver Sacks sets up a clinic to look into an unusually high incidence of colorblindness. Sacks is a brilliant, witty writer on neurological conditions (The Man Who Mistook His Wife for a Hat) but these essays also show him to be keen on botany -- and a good travel writer. His chapter on cycads is terrific.  (PAC44, $14.95)