If I had to say one text that is the fundamental one, I would go with a paper: “On Computable Numbers, with an Application to the Entscheidungsproblem” [1], by Alan Turing.
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But if you just started your course in Computer Science, then I will give you some “bookshelf advice" based on my experience. I had the pleasure to study all the books I am going to recommend you, and most of them I was able to read cover-to-cover during the university years. And they are also in my bookshelf for reference.
The book order does not represent rank of importance.
* “Introduction to the Theory of Computation”, by Michael Sipser.
* I also recommend the “Elements of the Theory of Computation”, by Christos Papadimitriou.
* Another good complement is “Introduction to Automata Theory, Languages and Computation”, by Hopcroft and Ullman. (Thanks, @reacweb, for the reminder.)
* If you feel you want to go deeper, and like a book which you cannot skip a single word, I strongly recommend “Modern Graph Theory”, by Béla Bollobás: it is one of my favorite textbooks ever!
* While a lot of people seems to praise either the Cormen et al. or the Sedgewick books, I have the feeling that “Algorithms” by Dasgupta, Papadimitriou and Vazirani is my choice for “fundamental” book. But I decided to
* “Modern Operating Systems”, by Andrew Tanenbaum.
* “Artificial Intelligence: a Modern Approach”, by Russel and Norvig.
* “Modern Compiler Implementation (in ML)”, by Andrew Appel.
I would like to also recommend “Concrete Mathematics”, by Graham, Knuth and Patashnik, but I remember to not feel it the most pedagogical book on the subject.
Good luck!
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[1]: https://www.cs.virginia.edu/~robins/Turing_Paper_1936.pdf