Thinking in Systems: A Primer, by Donella Meadows. More things I lived and worked on would had made more sense if I had read it a decade earlier.

There had been a lot books that were influential, but appeared not too long before I read them. But some had been around for long and were been widely known, so the opportunities for reading them earlier were bigger. The Selfish Gene is in that category.

The Child In You by Stefanie Stahl. It helps you to understand yourself better and also others. It gives you the vocabulary to explain things you feel or observe where you sometimes lack words for because you cannot locate why or how you feel like this. Its a great read for yourself, with parents or a partner to be more aware of one and another in different situations

"Behave: The Biology of Humans at Our Best and Worst" (Robert Sapolsky) I found it immensely helpful to understand about self and others, I have seen so many talented people ending up in awkward places due to lack of understanding of social dynamics.

Leaf by Niggle, Tolkien. If you worry about your life being a disappointment, or you are often distracted / lack focus, read this :-)