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".
I know this is an old thread. But I think it is important to point out that Modern Chess Openings is really only 122 pages of basic text style overviews of each opening, and then several hundred pages of variation. Obviously you should read an introductory book on chess and basic opening principles before reading a book like this. But if you just read the main 3 or 4 pages of text for each opening and move the pieces on a chess board you will get a concise overview of each major opening. Treating this book like it is an introductory chess opening book by skipping the variations pages means that when you get stronger at chess, or want to dive deeper into a particular opening you can do so using the same reference you started with.
I only bring this up because several people mentioned that he should read Fundamental Chess Openings instead. When my brother passed away I inherited about 50 of his chess books, and this was one of them. This book seems to be well written and does go into really good detail on each opening. But it is about 450 to 500 pages of explanation on various openings. So if someone is just starting out and is only 900 rated as the questioner is then reading the main text overview summaries from Modern Chess Openings is actually probably a better idea than reading the 500 page notation in the middle of a paragraph heavy fundamental chess openings book. And frankly I like the structure of the MCO book much better because it combines a basic overview with very well structured list of moves. And having the notes for different variations be written out to the side vs in a garb of text in the middle of a paragraph like in the FCO makes it much easier to read through, and then reference later.
But I will say there is a great series of articles on the simplify chess website on pawn structures that I am currently reading that are written for an intermediate level. These discuss the strategies and plans that come from the most common pawn structures. It is based on a similar outline to the book pawn structure chess, but is written in an intermediate style similar to the difficulty level of a Lev Alburt book. So there are lots of diagrams and only modest amount of notation between diagram, but the material is still very thorough and covers a lot of knowledge.
I inherited chess structures and pawn structure chess and both are two notation heavy and intro books like the chess players bible is too simplistic. This is the best in the middle difficulty level version I have found. I think at a minimum printing this articles off and reading them should improve the average rating of most club players by a couple hundred rating points. I wish he did the opening articles in the same manor, but most of them are more of an interactive view so you would have to copy and paste stuff into power point and then print out to get the information in a print out and re-readable reference form. I printed these pawn structure articles off and am going to read through that in addition to finishing the comprehensive chess course book series. But then I plan on reading though the MCO. My club does 25 minute rated games only every Friday. I waist a lot of time in the openings so at that time control if I could cheese people out of the opening I would likely gain at least a couple hundred rating points.