Fletcher Pratt & L. Sprague deCamp 
Wall of Serpents [EPUB ebook] 

The Mathematics of Magic was probably the greatest discovery of the ages — at least Professor Harold Shea thought so. With the proper equations, he could instantly transport himself back in time to all the wondrous lands of ancient legend.But slips in time were a hazard, and Shea’s magic did not always work — at least, not quite as he expected . . .The Wall of Serpents is the third in L. Sprague de Camp and Fletcher Pratt’s much-loved Compleat Enchanter series.