Are there any books that are considered a must for someone new to the field of philosophy?
I understand it is a vague question but it is a topic I have recently become interested in because of digging deeper into mental models. Mental models address a lot of 'practical' situations but I am realizing that they fall short when it comes to bigger questions of life.
What path did you follow to develop a personal philosophy?
Books aren't nearly as good because you won't get to discuss the ideas with other students who are also learning the subject, which is half the fun and half the point of philosophy. You'll also miss out on the insights and explanations of the professor, which will be very valuable, if the professor is any good.
As for books, the Socratic dialogues are probably the best place to start, since the ideas pretty easy to grasp compared to later philosophy, they're written in an engaging way, and give you plenty of food for thought. Also, much of later Western philosophy is a reaction to, comment on, or has been influenced by Socrates and Plato. You'll be much more "in the loop" after getting some familiarity with ancient Greek philosophy than if you just dived straight in to later work.
Something else you'll want to be aware of is that in contemporary philosophy there are two major approaches: Analytic and Continental. Adherents of these approaches generally despise one another, denigrate, or ignore one another's work, though at least more recently the Analytics have been starting to read, re--envision, and appropriate Continental thinkers.
The Analytic approach dominates philosophy in the English-speaking world (and is coming to dominate the rest too), and when you take philosophy courses that's the view you'll most likely be exposed to, and it's Analytic philosophers you're most likely to be recommended when you ask about philosophy, especially on sites like HN, which are more likely to be peopled by fans of logic, rigor, and science, which Analytics themselves are huge fans of.
But your exposure to philosophy would be incomplete and probably really biased if you were mostly exposed to Analytic thought or viewed philosophy primarily through an Analytic lens.