It's no secret that the Louvre's Venus de Milo is armless and that the Tower of Pisa has a mean lean of 5.5 degrees. But did you know that the maimed marble statue was originally a brightly painted niche decoration for a Greek gym, or that the campanile's famous tilt was once believed to be the architectural revenge of a bitter hunchback? These strange but true facts are highlighted in these book-length crash courses on the popular icons. However, only ''Disarmed'' will satiate voracious art buffs. Curtis, former editor of ''Texas Monthly,'' maintains authority with his in-depth, engaging analysis (though he occasionally overwhelms readers with minutiae). Meanwhile, journalist Shrady breaks no ground with his too brief, oft dry account. But if you judge a book by its cover, you may find a bias for ''Tilt'''s quirky slanted design.