Here's a list of who had read the books before being cast in the movies and who had not. I'm sure most if not all have eventually read LOTR, but the ones on the HAD READ IT list are the ones who had read it before the movies were ever mentioned. :-)

HAD read it	Had NOT read it
Sean Bean Jørn Benzon Ben Britton Nathan Clark Jonathan Harding Bernard Hill Christopher Lee Alexander Lindsay Ax McClennan Dominic Monaghan Karl Urban	Sean Astin Billy Boyd Brad Dourif Sean Fitzpatrick Bruce Hopkins Ian McKellen Viggo Mortensen John Noble Miranda Otto Andy Serkis Liv Tyler Hugo Weaving David Wenham Elijah Wood
Read part of it	Unknown
Orlando Bloom Craig Parker	Cate Blanchett Ian Holm John Rhys-Davies