I posted recently that I am reading The Great Old Ones: The Complete Works of H. P. Lovecraft. It is a daunting read. I've read over half of it, but it is a little like trying to read all the 1001 Nights in one sitting. The dark and dreary stories can seem repetitive over time, so I take a break.
During the current break, I am re-reading C. S. Lewis's Space Trilogy. These books helped inspire me to write my newest book, Lightning's Edge: Lost World Odyssey.
The archaic language of both Lovecraft and Lewis can challenge me. Lovecraft's prose verges on poetic at times, while Lewis's seems more straightforward and less flowery.