How to Read and Why, by Harold Bloom.
As someone who loves to read, but doesn’t have a background in the humanities, I found this book useful in opening up a range of other titles to consider. Bloom clearly thinks that Shakespeare was the best author in history, and this thesis recurs throughout the book, but even if you disagree on this, the book is a challenging read (keep a dictionary by your side while you work through it), but a rewarding one.