Founder of the website/blog IntrovertDear.com has a book about...introverts! Surprise surprise! Jenn Granneman writes about what introversion is, how to manage self-care, navigating friendships and work relationships, how to cope with interviews/jobs and more. If you're coming to terms with being an introvert (or you're extrovert who wants to know more!) this isn't a bad pick.
I am not a huge follower of the blog so I don't know how much is cribbed from it or how much alike the book and blog are. I went into this thinking that it was another "Introvert 101" books and I still believe that is what it is after finishing it. If you're familiar with introversion then a lot of this is probably not that strange or unfamiliar to you. You may pick up one or two or more helpful tips or insights but unless you're entirely new to the concept there's probably not a lot here you haven't heard before.
But I'd say that it's also for extroverts, too, if you want to better understand an introvert. The title might seem like Granneman is offering up mystical secrets to unlock the mystery that is the introvert but really, it's her just telling people who introverts are and what they do and how to treat them (and how introverts can better utilize their strengths in all kinds of relationships, personal/romantic/professional/etc.).
I will say that not everything will apply. I wasn't interested in the dating/relationship chapters at all, so your degree of interest may vary. Sometimes the book did feel a bit repetitious or some of the topics (like the dating/romance) didn't really need more than one chapter or even an entire chapter dedicated to it. But it may depend on what you're looking for and what info you need.
I'd say this is a new go-to book for introduction to introversion but if someone is already familiar with Granneman's website, it might not be for them. I'd also recommend 'The Introvert Advantage' or 'Success as an Introvert For Dummies' (yes part of the 'For Dummies' series) if you're looking to understand introverts and introversion better.