I study IR and I think these books are useful in terms of explaining some fundamentals of international relations theory:
International Relations: Critical Concepts in Political Science - Andrew Linklater (this one is particularly well-rounded and will explain all the IR theories that you will take in your undergraduate years)
The Anarchical Society: A Study of Order in World Politics - Hedley Bull (very popular in academic circles)
The Constitution of Society: Outline of the Theory of Structuration - Anthony Giddens (extremely popular academic but I, personally, disagree with many of his positions)
The Claims of Culture: Equality and Diversity in the Global Era - Seyla Benhabib (she makes some extremely interesting points)
The Transformation of Political Community - Andrew Linklater (Linklater is a paradigm-shifter, love his stuff)
Introduction to Critical Theory - David Held (a personal favourite, Held is a legend but the hardest and most complex on this list)
These are very academically oriented books and might be hard to crack for someone who just entered into the IR arena but you're bound to come across them in your education. I say if you find some difficulties, you can message me here or just push through it and do some supplementary research. They were tough for me at the beginning when the concepts were new but now it's all good.