I just ordered the FSI Serbo-Croatian course on Etsy (the books only.) It seemed like a rare find that I couldn't pass up. They were published in 1965 & 1968. Funny how Volume 1 is so worn compared to Volume 2. Maybe the previous owner was really gung-ho, then gradually lost interest, barely touching the second volume. I'm always curious to know the history of these old language books I have: Who owned it? Why did they choose that language? Did they make much progress?, etc. Anyway, this course is available online as well, but I prefer having the actual texts, though the recordings will be helpful.