Campbell biology
Downsides: if you're only looking for a cursory introduction to a few themes, it's probably going to be too thick (weighs in at ~9 pounds) and too expensive (> $100).
I don't know that it stands head and shoulders above comparable texts, but it does what it sets out to do.
I believe it comes with a study guide which can be useful for self-study.
If the cost of a book is unappetizing, MIT's Intro to Biology on edx is great and free.