Bertrand Russell is one of my favorite philosophers. One interesting thing about him is that his philosophical treatises, while they are written almost entirely in prose, still retain a great deal of mathematical (i.e. precise) reasoning in them (This is probably because Russell was not just a philosopher, but also a mathematician). For this reason, readers who are familiar with STEM subjects (e.g. math, science, and engineering) will find his writings a lot easier to understand than those written by other philosophers.
This is perhaps the best primer to philosophy in general (maybe not for humanity-related areas such as ethics and aesthetics, but definitely for metaphysics, epistemology, and ontology). In this book, Russell introduces some of the most common questions in philosophy such as the conflict between idealism and realism (i.e. "Matter is a byproduct of mind" vs "Mind is a byproduct of matter"), the definition of "idea" in Plato's metaphysical worldview, and so on. Then he even proceeds to explain his endeavors to define the whole of reality itself, as well as ways in which we may induce it from our sense-data and their apparent relations. (You can read it Here)
This one is significantly more challenging to read than "The Problems of Philosophy". Nevertheless, I strongly recommend this book due to its interdisciplinary approaches to the study of psychology, cognitive science, and problems of epistemology in general. Although there aren't any diagrams in this book (which is unfortunate because I think it would have helped a lot), the reader will probably be able to realize that many of the concepts which Russell introduces here can be illustrated in terms of graph theory (i.e. where ideas are denoted by vertices, and associations between ideas are denoted by edges). For example, according to Russell's reasoning, the difference between a "vague" idea and an "abstract" idea is that the former is an idea which is closely associated with ideas that are not so closely associated with each other, whereas the latter is an idea which is closely associated with ideas that are also closely associated with each other. This is one example, but there are many more, and they can all be rendered graphically using points and lines on a sheet of paper. (You can read it Here)
This is probably one of the easiest and most relaxed books ever written by Bertrand Russell. Unlike his other treatises, this one focuses on everyday human emotions and their hints on morality, rather than topics that are mathematically engaged. So if you are more of a fan of humanities than purely abstract concepts (such as those in metaphysics), I recommend reading it. It feels a bit like C.S. Lewis's "Mere Christianity", although Russell tends to speak of ethical topics from a more secular and rational viewpoint. (You can read it Here)