# brought their books

"collected their books." The word "books" refers to scrolls on which magical incantations and formulas were written.

# in the sight of everyone

"in front of everyone"

# the value of them

"the value of the books" or "the value of the scrolls"

# fifty thousand

"50,000" 

# pieces of silver

A "piece of silver" was the approximate daily wage for a common laborer.