Einstein's Greatest Mistake: A Biography
It's perhaps odd to describe a book about general relativity and quantum physics as being "written for young adults," but that's the sensation I kept getting while reading Einstein's Greatest Mistake. David Bodanis explains everything in simple language--even such common terms as "light year" are glossed in short words. I suppose that it's probably a Scylla-and-Charybdis problem. On the one hand, Bodanis doesn't want to write a popular-science book; he wants to write about Einstein the person, and how Einstein thought, and how his enormous genius could lead him to spend the last twenty years of his life barking up blind alleys. On the other hand, he can't do that without explaining the scientific breakthroughs that made Einstein so (over-) confident in his genius in the first place.
There's not a lot that's new here for anyone with a decent level of scientific literacy, but it's still a good story. The story in short: Einstein reluctantly included a clunky fudge factor in general relativity in order to conform with the experimental evidence. Ten years later, it turned out that the experimental evidence was incomplete. Einstein, his intuition vindicated, doubled down on it by never accepting quantum theory.
Einstein's Greatest Mistake lays out the whole sequence clearly and convincingly. It's a good book if you want to get a fairly qualitative yet still useful introduction to Einstein as a thinker. Personally, I'm going to check out the author's website, where he promises a 22,000-word outtake that goes into the science in more detail. But that's just me.
I thought Bodanis's earlier book, E=mc2: A Biography of the World's Most Famous Equation, did a good job of pulling the pieces together. (That's special relativity, which--believe it or not--can be understood pretty well using high-school algebra.) Walter Isaacson wrote a very fine and readable biography of Einstein back in 2007; it's well worth reading.