Having grown up bilingual and studied linguistics, I thought Wyner had a fantastic balance of practical tasks/suggestions and the science behind them — plus emphasis on efficiency over grinding (work smarter not harder), and some genuinely funny jokes too — even phonetics and IPA, which I've struggled with since my first Intro to Linguistics class, is presented in a really approachable way.

That makes this a really useful and (I'm anticipating) effective resource to customize your language learning experience based on your goals, experience and comfort levels, target language(s), etc. There are modifications for first-time learners, languages with different writing systems (such as Chinese and Japanese), people looking to refresh intermediate/advanced fluency, nerds like me who want to dig deeper into particular aspects of language learning.