I’ve read at least five C.S. Lewis biographies over the last few years, plus several groups on the Inklings.
Too often, books on Lewis are either very accessible but basic, or insightful but dense and hard to get into if you’re not an academic.
One of the books that does a great job of being brief but still insightful is James Como’s C.S. Lewis: A Very Short Introduction:
Como, a founder of New York’s C.S. Lewis Society, uses years of research to inform this book, yet manages to boil down to the essentials and avoid too much speculation.