Here's the set-up: Spenser and Susan are sitting on a park bench, and Susan says there are parts of Spenser she knows nothing about. For example, his childhood. What was he like as a kid? So Spenser tells her a few episodes from his childhood. The chapters with this info are interspersed with chapters from the park-bench present, with Spenser and Susan analysing the incidents, which include the titular bear, a couple of fights, and an early instance of Spenser rescuing a young woman in distress.
There's no real plot in the book. Spenser tells about something. He and Susan discuss it. Spenser tells about something else. He and Susan discuss it. And so on.
If you're a faithful reader of the Spenser novels, as I am, you may know some of this stuff already. Spenser is raised in a manly household by three manly men, his father and two uncles, who teach him to box and who teach him his code, mainly based on Davy Crockett's "Be sure you're right, then go ahead" philosophy. Also, "there's law and there's justice, but they're not always the same." The uncles are also good with their hands and do carpentry and construction work. So does the young Spenser.
But if you're a faithful reader of the Spenser novels, you might be taken aback by the revisionist history (I told you I'd get to it). At the end of the book, we find out that when Spenser goes to college, he plays football for a couple of years, injures his knee, and drops out of school. He takes up boxing and is pretty good (but not great). He joins the state cops, and the rest is (revised) history. What happened, you faithful fans might be asking, to Spenser's service in the armed forces in Korea? It's disappeared. I can see why, of course, but it's a little odd to hear nothing about it.
One more thing. Who's the audience for this book? It's supposedly aimed at young readers, but do they know (or care) who Spenser is? Do they want to "see how the legend began"? The writing in the book is little different from any Spenser novel, and at 169 pages of big print, it's really not that much shorter than any of them, either. I suspect more adults will read this than kids. But what do I know?
I read it, I enjoyed it, and I've already spent more time writing about it than it took to read it. If you're a Spenser fan, you'll read it, too. If you're not, you won't, I'll bet.