Today a king who wants a mistress must abdicate his throne and marry her; and with presidents, the mistress pattern ends, for their love affairs are not permitted to exist historically at all."" With that shaft at contemporary hypocrisy, the author launches a detailed discussion of some of the 19th century hereditary rulers who married their queens but loved their girl friends. Julie de Krudner, a middle-aged religious visionary, brought Russia's eunuchoid Alexander I to his knees in prayer (probably accounting for his painful case of housemaid's knee) and instigated the shortlived Holy Alliance. Ludwig of Bavaria warned off a remonstrating bishop with, ""You stick to your stola, I'll stick with my Lola,"" married the melodramatic Montez and bequeathed her a royal, fatal case of syphilis. Napoleon III rivalled Edward VII as master of continuous dalliance and Tsar Alexander II fell in love with a 10 year old, seduced her when she was 17, maintained her in his palaces together with their children and finally married her when his Empress died. Leopold I of Belgium was a royally dirty old man who wound up with an ex-prostitute and the Serbian princess arranged their affairs like musical comedies. Franz Joseph brought grave dignity to his passionate liaison with a plumpish actress and the story of Rudolph is told again. Highly entertaining collective biography, lightly written but carefully researched.