''Spiegel'' is an engrossing, straightforward account of a mogul and his movies. Mr. Sinclair (who has also written a biography of John Ford) succeeds in conveying the power, intelligence and corruption of the man. And the charm? Mr. Sinclair reports that Katharine Hepburn spat in Spiegel's eye after shooting ''Suddenly Last Summer.'' ''It's rather a rude gesture,'' she later said, ''but at least it's clear what you mean.'' I'm with her.