Learning from lectures alone is very slow. Learning from books alone is faster. Learning from both books and lectures at the same time is very fast.
Edit: Since this answer is apparently unpopular, let me explain it a little bit more clearly. I have a PhD in Chemical Engineering and taught college-level chemistry for a few years, so I know something about how people learn difficult material and what makes a class effective. I don't have research data to back up these statements, so it is possible that other people would have a different experience. However, I have never met a person who did. I don't mean I have never met a person who did well in school without going to lectures, or without reading the book. What I mean is, I have never met a person who did not learn fastest from both lectures and the book.
Yes, you can learn via either method alone, and different people tend to learn more easily from one or other. It is also possible that a good lecturer can be more effective than a bad book, and vice versa.
My statement assumes that the quality of both the lecturer and the book are good (or at least equivalent), and is about efficiency - how much time/effort does it take to learn something. The reason books are more efficient than lectures in general has to do with information bandwidth. A book can communicate more complex material in a shorter time than a lecture can. The reason that lectures in conjunction with books are the most effective way to learn is that a good lecture can tell you what parts of the book to focus on - a good teacher points out the most important details. It also gives you a different view of the same material, and reinforces memory through repetition.