It would be interesting if we answered the question, along with another: why/how? (Is that two other questions?). Knowing why and how, say, Roger L'Estrange's 1715 Aesop's Fables influenced you so would be much more interesting than just knowing that, it did influence you.
Me? Jane Jacobs' Death and Life of Great American Cities. I read it when I was 21 (1998) and it changed how I walk down the street, every single day of my life since. Why this street is full of life, and that street is empty, for example. But not only that: she basically argues that central planning is disastrous for cities, and that cities should think on the street and human levels, from the ground up, and then iterate from there (rather than top-down). These ideas applied to other areas of life -- like software development, for example -- have been very powerful for me. I'd argue that Jacobs' thought is a precursor to the whole XP/agile software movement, for example. Both start with the same premise, the spec is dead (in software language). All this is probably obvious to y'all, but wasn't to me at 21 ;)
"Influential" can be tricky to pin down, but a couple of books that made a big impact on me at a certain age were "Prometheus Rising" by Robert Anton Wilson and "The Muse in the Machine: computers and creative thought" by David Gelernter. (The latter arguably counts as a technology book, since it's supposed to be about AI, but it's more to do with psychology and literature.)
More recent sorta/kinda philosophy: "The Craftsman" by Richard Sennett; "From Novice to Expert: excellence and power in clinical nursing practice" by Patricia Benner; "The Craft of Thought: meditation, rhetoric, and the making of images, 400-1200" by Mary Carruthers.
I also like to think that "Asterix in Britain" by Goscinny and Uderzo had a formative influence on me...
Book recommendations and tastes are very personal.
I suspect that if you're not specifically looking for technical books you'd be better off looking at the common lists of 100 most popular books, or similar.
Me? I'd say "Dune", but that is only useful if you have an interest in sci-fi. If not then the Count of Monte Cristo is a glorious tale of revenge and plot. Beyond that recommendations become vague: Steven Brust, Zelazny, Tolkien, Pratchett & etc.
The Bible. Harold Bloom's "Western Canon", Knausgaard's "Min Kamp", "The World According to Garp", Robertson Davies Deptford Triology, "Anna Karenina", "Consilience of Knowledge", "House of Leaves" "The Apprenticeship of Duddy Kravitz" All these books messed me up to varying degrees.
The Essential Rene Guenon (meta-tradition). Last and First Men by Olaf Stapledon (Spinoza-inspired scifi). Embassytown by China Mieville. Past Master by R.A. Lafferty.
Aesop's Fables by Roger L'Estrange, 1715 3rd ed. in 2 volumes, ~900 pages total, otherwise it's an abridged edition.
The book "The Age of Reason" by Thomas Paine really convinced me that something like a personal God doesn't exist. I've been raised a Christian, so changing the Christian mindset was not easy for me, it took a course of several years to stop believing in a personal God.
My favorite picture of god was Einstein's -- or Spinoza's, which is where Einstein got his idea: a god that isn't remotely like a religious person's idea of god, an entity roughly equal to nature, and one that couldn't be bothered with our small problems.
When I see something in nature that speaks to a deeper level, that connects apparently unconnected things, that's as close as I get to a religious experience (i.e. not very close at all). For example, there's a cicada that has evolved a survival strategy based on prime numbers:
I always like finding examples that connect mathematics with nature, so this one was particularly satisfying.
My point? I can easily picture natural selection brutally filtering out everything that didn't make an ingenious (and chance) use of prime numbers to assure survival, but by the same token I can't picture nature caring at all what I think.
Me? Jane Jacobs' Death and Life of Great American Cities. I read it when I was 21 (1998) and it changed how I walk down the street, every single day of my life since. Why this street is full of life, and that street is empty, for example. But not only that: she basically argues that central planning is disastrous for cities, and that cities should think on the street and human levels, from the ground up, and then iterate from there (rather than top-down). These ideas applied to other areas of life -- like software development, for example -- have been very powerful for me. I'd argue that Jacobs' thought is a precursor to the whole XP/agile software movement, for example. Both start with the same premise, the spec is dead (in software language). All this is probably obvious to y'all, but wasn't to me at 21 ;)