Immersion. Books or apps won’t teach them anything.

Here are some resources:
Input hypothesis
Tatsumoto’s Guide to Learning Japanese (or any other language for that matter, just skip the kanji specific parts, and I wouldn’t recommend joining their community)
Antimoon’s Learner Reports

I had already written at length about the topic, but the OP I had replied to seems to have deleted their post so I’m just going to do it again.

My native language is Turkish. I reached basic English fluency when I was ~14 years old and I’m currently doing Japanese immersion with my comprehension rate of the Japanese content I consume being around 90% (mostly video game content and anime). I achieved this primarily by consuming interesting content in my target language. I didn’t go to any language classes at all in both cases, and school itself likely only made my English skills worse.

This technique essentially aims to replicate how people acquire their first language when they’re babies, which essentially means lots of comprehensible input and no output initially. Input comes first, output comes a few thousands of hours later, similar to how it takes 4 years for a human baby to have acquired the language just enough to be able to start speaking. How language acquisition itself works can be explained like this.

Though in the case of adults this can take much less time with the help of flashcards, dictionaries, and reading (you should not start reading from the get-go though). And getting that much input is thankfully much easier in our age because of the Internet, and essentially all you need to do is watch interesting, comprehensible (visual cues help a lot) content in your target language. You should aim for 10,000 hours of comprehensible input for basic fluency, which would take around 4 years at 7 hours per day, or more unrealistically 1.5 years at 18 hours per day.

I haven’t yet come across any guide other than Tatsumoto’s that promotes using only libre tooling for language acquisition, so that’s why I primarily recommend his website.

Community colleges may offer cheap foreign language classes.

Not a book, but check if your library has access to any learning apps for free… or books for that matter.

If you visit enough russian warez sites you will learn what Скачать means. Usually that’s the second language of the pirate world after broken english.

For books my first stop is Anna’s archive

Not really about piracy, but doing language exchanges is a great way to learn a language. You teach someone your language, they teach you theirs. It’s free and it gives you more knowledge than any book. Natural languages don’t work with strict rules usually, natives speak by feel, the only way to learn that feel is by interacting with people.

Look into Dreaming French (originally Dreaming Spanish but they recently extended to French).

What is the book called? I can try searching for it.

As for learning languages, I find that a combination of structured lessons (from an instructor or recommended course book) with speaking practice (with someone you know or through a language exchange program) works best. Other things that help are watching films or series in your target language, listening to music and translating the text.

I just hung out with spanish speaking people and used duolingo (back when it was good) until I could speak and understand spanish. That way takes years though. I would recommend including structured classes for anyone who isn’t an auditory learner.

There are a lot of language learning resources on the site i use. I did a whole Michel Thomas advanced audio course for portuguese that helped me learn the basics. What’s the title of the french book your friend is looking for?

Just use the AirLearn app. I’m learning Spanish on it. Pretty good. If you use an Android, there’s also a premium APK mod for it.