I just picked up a used copy of "BLACKSAND!' a add on for the original version of Advanced Fighting Fantasy. This copy is far from used, its spectacular! Here's a few things that struck me.
First bit of the book is new monsters, skills and spells. To which I may not use most of them, and I believe they are all included in the new core book anyhow.
The second bit is on building your on towns, cities etc. this is friggin awesome! There are a bunch of random tables for determining all kinds of things. This in combination with the 5e DMG your gold. Honestly your pretty much gold with this book. I can't wait to start working on something new. The idea was to help you expand the world of titan. As there is not a lot of things set in stone per se, as far as maps go. I remember fondly as a kid with this book and the other few mass writing adventures, and locations. Honestly I get it.... I get why Hollywood keeps re doing stuff we liked when we were kids. This is the exact feeling I had when I cracked the book. And now I'd addicted.
The third bit of the book details blacksand, and OH.MY.GOD is it awesome. There's little bits about each section of the city, with just enough info to get you going and adventuring, but not enough to be super railroadey.
The rumors table is quite genius. As fighting fantasy was always d6's only. The rumor table goes from 11 - 66. So the gist you roll one dice and that determines the first number, then you roll a second dice. So for example I got a 3 on my first one and a 6 on my second one. Excellent rumor #36.
The last part of the book is a murder mystery adventure. It is quite railroadey, but I think it would still be a lot of fun. The way the adventures for AFF were written it was more like a movie and the DM was the "director". Its nice that they have a "props" section for each scene.