Match the book to the problem
Atomic Habits is useful for routines, Deep Work for focus, The Power of Habit for behaviour loops, and Think Again for changing your mind. A good match matters more than a popular title.
Avoid workbook confusion
Self-help searches often return journals, workbooks, summaries, and study guides. Those may be useful, but they should not replace the base book unless you searched for them directly.
Compare format before price
Paperback, hardcover, Kindle, audiobook, and international editions can all appear together. Check the format before assuming one store is cheaper.