When I was thinking about getting a PhD I looked thru a couple different books about the subject. "Getting what you came for" by Robert L Peters was the best overall book of practical advice at the time. (edit: Feibelman's "A PhD Is not enough! A guide to survival in science" was pretty good too.)

Years later I found the Stanford Encyclopedia of Philosophy's entry on the Scientific Method to be very interesting: https://plato.stanford.edu/entries/scientific-method/ especially if you're a scientist or engineer. But don't let it distract you too much - you typically don't have to think about things at this level, it just answered a bunch of meta / "why are things this way?" questions I had.

I have a bit of a background in the humanities, and I found Stephen Ramsay's "Reading Machines: Toward an Algorithmic Criticism" to be a very insightful take on the role of humanities in our technological / information-oriented world.

When I first got involved in research as a programmer, and later towards the end of my PhD, I read "Middlemarch" and I still look over parts of in now and then. It has a strong set of themes on intellectual/artistic/political yearnings for achievement and paths towards failure. And it's just beautifully written.

Good luck! Let me know if you need any more advice!