There are a large number of books, websites, apps, videos, and audiobooks that you can use to prepare for taking the exams and getting your amateur radio license. Everyone will claim that their method or resources are the best ones to use, but really any of them will be fine… It just comes down to how you learn best.
HamStudy.org is a website, phone, and tablet app where you can practice exam questions for the US and Canada amateur radio exams. The HamStudy.org website is free, but the mobile apps do have a low cost.
HamTestOnline is a website-only application where you can practice exam questions and do practice exams. HamTestOnline has a subscription option for gaining extra features, but the base application is free.
Geared specifically towards learning the minimum amount of information necessary to understand an exam question, the “Fast Track” series of books comes in traditional, ebook, and audiobook formats. It includes some explanation of the why on topics, and also includes humor and tricks to remembering the answers.
For the most concise, and fairly terse, way of preparing for the exams, “The Easy Way” series of books provides virtually nothing other than the question, correct answer, and a sentence or two about why or how to remember that answer. It's similar in concept to websites like HamStudy.org, focused on memorizing answers to pass the exams.
For the most comprehensive single volume books to actually study the topics in each exam, not just focusing on passing the exams, ARRL provides a manual for each exam. While some people may feel that much of the material in the books is irrelevant to passing an exam, the manuals do provide more complete summaries of each topic area.
David Casler YouTube channel covers a variety of topics related to amateur radio. At the moment, his Extra class questions are not up-to-date, however his General and Tech questions are current. While often more in-depth than necessary just for passing the exam, these videos are effectively a free exam prep class, and are similar in depth to the ARRL License Manuals.