: Websites like Project Gutenberg, Open Library, or Google Books might have relevant public domain or Creative Commons-licensed materials. You can search with specific keywords like "English language learning PDF" or "assimil English PDF."
The Assimil Philosophy
You can buy a used copy of the book for $10-$20. The older editions (without the digital code) are dirt cheap. You will miss the audio, but see step 2. assimil+perfectionnement+anglais+pdf+free