Hi @nerdlingwrites - answering your question this way because I would have been curious too and this way more people can see it!
It depends on the publisher - some give the artists a manuscript/rough, others don't, but even when they do, it's not required reading (that could take a lot of time after all!).
Personally I like it! Generally speaking I only read a few chapters to get the feel of the story/characters and see if anything sparks an idea (and then I read the rest after the job is done, at my own pace, because: book!!!!), but the brief the Art Director gives an illustrator usually has all the necessary information (including spoilers sometimes).