The book "Let Over Lambda" by Doug Hoyte, which explores the advanced features of Common Lisp, is now freely available online. First published in 2008 to coincide with the 50th anniversary of Lisp's invention, the book focuses on macros, programs that write programs, and their ability to enable abstraction, productivity, and code efficiency. Written for experienced programmers looking to push the boundaries of programming knowledge, "Let Over Lambda" offers a unique and entertaining perspective on the capabilities of Lisp and the art of programming itself, making it a valuable resource for those seeking to deepen their understanding of the subject.