Over the last 5 years, I've been actively pursuing languages as a hobby more thana passion. I'm fron a Spanish speaking country, with a bilingual upbringing, thanks to my parents speaking English during my formative years. I took a class on Esperanto, a class in German and I'm currently doing a French course for the A1 test. But over my experience with both old and new languages, I can say that most of them are related in one way or another. You can pick up words that mean the same, and even similar structures that you can user to learn them. The book you mention sounds like something I should be picking up as we speak haha, thank you for bringing such an interesting subject up, looking forward to read more about it!

PD: I'm posting all of my French course online. I'm hoping to entice someone to explore languages freely, not as an obligation, but out of curiosity and fun for the learning process.

-A.