Reading provides a lot of varied benefits, many of which could be helpful in most jobs.

• Dealing with people -- reading (particularly fiction) can help you become more empathetic. And reading can expose you to a broad range of topics and interests, allowing you to connect with more people by potentially being conversant in (or at least aware of) their interests
• Communication -- as noted by many here, reading a lot helps improve vocabulary, spelling, grammar, and writing in general, all of which can improve your written communication. Vocabulary and some writing skills (like organizing your thoughts logically) can also help oral communication.
• General knowledge -- even if you only read fiction, you're likely to learn many facts (many authors research extensively to provide authenticity and accurate depictions of real-world elements in their fictional worlds). And of course, reading non-fiction will often provide lots of knowledge on the non-fiction topic. Having a broad base of general knowledge could be helpful in your career (even if it has no direct bearing on your job -- if it just makes the boss/coworkers think you are smart, that can be to your advantage.)
• Specific knowledge -- Non-fiction related to your field obviously is likely to be helpful to your career. And more general reading on business, success, or personal improvement can be helpful as well.
• Focus and concentration -- reading can improve these abilities, which obviously can be helpful in most types of work.
• Creativity -- Increased creativity is a widely touted benefit of reading, which can be helpful in many lines of work.
• Critical thinking and problem solving -- engaging with diverse ideas, perspectives, and arguments -- which reading can provide -- can improve critical thinking. And of course, these skills are valuable in a wide range of careers.

Obviously, what you read has an impact on how much of the different types benefits you get -- for instance literary fiction is more associated with empathy, theory of mind, critical thinking; non-fiction for directly applicable knowledge in your field or broader business knowledge. But I would not recommend putting too much focus on reading for your career -- that can certainly be a goal and an influence on your reading choices, but take care to not making reading a chore, or an (unpaid) extension of work. Get out of your comfort zone from time to time, but also read what you enjoy.

Personally, I believe that reading has helped me with all of the above, and helped my career as a result. And there may be more things to add to the list. I just went with some of the most commonly cited benefits.

I guess it would depend on how the story develops whether I would be amused or disappointed by an ending like that. If there were comedic elements throughout, and we got the sense that the protagonist was not really a good detective (and/or an unreliable narrator, if from a 1st person POV). I kind of like the idea of taking the trope of a brilliant amateur detective who is much more competent than the police, and turning it on its head. As I said, though, it depends on the execution.

Spoiler: >!Everything is fictional. Nothing is real. You are the only being who exists at all, and everything that happens/has happened/will happen is all in your mind. At least, that's one philosophical concept of the universe.!<

&nbsp;

While it is true that fictional works are tales of events that didn't actually happen, the same is probably true with at least parts of memoirs and autobiographies.

And really, does it matter if it happened? If you read real, true, completely accurate accounts of battles, or journeys, or other events that occurred thousands, or even hundreds of years ago (or during your lifetime, some distance from you) could you really tell if it was fact or fiction? Would the story be more or less impactful or meaningful based your belief the events occurred as described?

Another point to ponder: there is truth in fiction. Or at least, that's what many (most?) authors strive for: expressing truth through storytelling. For instance, most people will probably agree that doing the right thing is important and good, even (or especially) when it's hard to do. A common theme in HP, as I'm sure you know, and many other stories. And of course, there are many other true things expressed in fiction. So despite people and events being imaginary, there is still a lot we can learn. In fact, reading is a pretty good simulator for real experiences -- research with fMRI (if I recall correctly) shows that the same areas of the brain are active when reading about something as when experiencing it.

And as /u/Petal_Chatoyance said, suspension of disbelief is powerful and important. Try to nurture it, and re-develop it if necessary. I'm not sure how you do that -- maybe just tell your brain to chill when you're trying to enjoy some fiction, and it starts mumbling that this isn't important because it didn't happen (in our reality). Or maybe take a break from fiction and enjoy some non-fiction for a while. Or try some less speculative fiction (mysteries, thrillers, adventure... things set in our world, or at least adhere to the same limits of technology, physics, biology, etc....) It may be easier to suspend disbelief is there is nothing too fantastical about the story. Maybe try things like that until you start craving something more fun or escapist.