I don't know if Orlando Furioso is the most over-the-top book ever written, but it may very well be. It is arguably the BEST over-the-top book ever written. Ironically, it has inspired some truly awful B-movies. In fact, a plot synopsis would read like a Readers' Digest compendium of ten seasons of the silliest of Saturday morning TV shows. The characters, though numerous, are often interchangable and for the most part disposable. That said, Orlando Furioso is truly unique and a very enjoyable read.
The key, of course, is the author. Ludovico Ariosto was a wonderful poet with a boundless imagination and astounding quality control. The poem, first published in 1516, predates television by more than half a millenium, but plays to the short attention span perfectly. His goal was to entertain, with never a dull moment. If there is a dull moment in the poem, the reader need only press on for a short while, because entertainment- perhaps even brilliance- is sure to be only a paragraph or two away.