Help on reading Nick de Firmian's Modern Chess Openings 15?

I used to own MCO. Back when it came out, it was considered a "good" book, although not the easiest to read. It was more of a short, condensed version of the multi-volume ECO (Encyclopedia of chess openings), all of this stuff before the days of databases. (Chessbase for DOS was still being used back then and there were digital CBF format versions of the Informants).

I would not recommend MCO at all these days. You're going to be dealing with 30 year old opening theory, many lines which are completely obsolete or refuted (Just look at some of the Nadjorf lines shown!) It's far better these days, with respect to older books, to just read a book that discusses in depth strategy and structures, rather than dealing with fashion. That's why something like "My System" can still be a good read, even if some of the lines shown can make you go "huh".