Probably I Capture The Castle by Dodie Smith. I have about half a dozen special editions of it at this point. It's sad, dreamy, and incredibly funny, and Cassandra, as a narrator, captures everything so well but so clearly through her own point of view. In one part she's looking over her own poetry with a critical eye; she described a bloom as a "frank-eyed floweret, kitten-whiskered", and says it "sticks in your throat like fish-bones". I love this description, because it shows how writing is tactile and sensory. 

I also loved Sherlock Holmes and Poirot books from an early age. I used to listen to books on tape at night because I was scared of the dark, and I think my parents thought they made me feel less alone. But really I thought it would trick the monsters under the bed, who were of course very real, but would be too scared to come out if they thought Sherlock Holmes (a noted martial arts expert!) was there.

I just today finished The Bells of Westminster by Leonora Nattrass. She writes great historical mysteries where a slightly hapless protagonist is embroiled in trouble, but sort of in the corner while everyone else is focused on larger events. Her books are almost as much a comedy of manners as they are mysteries and I think she should be selling trillions of copies. The Bells of Westminster is the diary of the daughter of the Dean of Westminster Abbey and she’s not quite as clever as she thinks she is - although she’s cleverer than the majority of men around her, many of whom are there to open the tomb of Edward Longshanks. Cue the kingly body going missing and some mysterious deaths…

I take my dog on three walks a day and sort of talk to myself while I’m doing it, which helps me undo knotted up plot problems. But really the most inspiring thing is the people around you. Being a writer is a lot of noticing things. Whoever I go anywhere on the train I look at people and the clothes they’re wearing and what kind of sandwich they bought at Tesco and try to figure them out. And I’m probably wrong a lot of the time, but that’s why I write fiction rather than non-fiction. Normal people are quite extraordinary though. I know an old man who has, in his final years, elected to replace milk with Baileys. You couldn’t make it up, and, fortunately for writers, you don’t have to. 

This is a very old hat one, but as well as a reusable water bottle, I have a reusable thermal cup for tea and coffee (and hot chocolate). A lot of cafes will make your drink in that now, which is great.

Just remember that everyone else is making life up as they go along as well, even very important people. Also, probably don’t go out with that guy, it’s way more trouble than it’s worth (handily applies to whichever younger self you choose).