Practical Common Lisp is not a good option as a reference book due to the poor index. Some concepts that span a page or two are not indicated in the index.
Unlike many code practice websites, Exercism requires the user to develop and test entirely offline, submitting only the finished code. This promotes familiarity with essential tools and workflow, not just the bare language.
Rather than merely test for code correctness, Exercism uses peer review to improve general programming techniques. Users are encouraged to comment on others' solutions, and refine their own based on feedback.
Exercism requires using a CLI utility to fetch and submit exercises. This is inconvenient compared to web-only alternatives, and poses an additional barrier to entry for some users.
It's best to have at least a basic understanding of programming before tackling the exercises on exercism.io. You also have to know how to set up your development environment, as you will be coding everything locally (versus websites that provide you an editor right in the browser).
The book starts off fairly slow and easy to grasp. However after the first few chapters, the author starts including new terms and concepts that had not been introduced/explained.
This book will give the reader a solid understanding of programming fundamentals. All concepts are very well explained using diagrams, exercises (with answers provided), analogies and more.