I don't know FCO, but I think the time for books like that is almost past.
Decide on a repertoire, and if you feel the need for books, get some opening monographs about the openings/lines you want to learn. If you just want lists of moves you can get that for free from a good opening database.
I was under the impression that books like MCO and FCO are more than lists of moves -- that they provide analysis of what's going on as well. After reading a bit more on Amazon, I'm leaning toward FCO -- MCO is criticized as being full of typos, and FCO is praised: "For a beginner or intermediate, this book will give you enough theory to get started - your first half dozen moves with some deviations - but not enough to distract you from tactics and endgames."
I'm looking for a tool that will walk me through opening lines and point out the why as much as the what. Does FCO do that? Will an opening database do that as well?
Well, an opening DB has the advantage that you can click through dozens (or hundreds) of games. I find this an effective tool for learning an opening. After 5 or 6 games you start knowing where the pieces go.(and if you play through the game you often figure out why too) But maybe for someone else a book is better, I just find some authors a heavy read, and some books I can never get into.
Nunn's Chess Openings has a great reputation for explaining lines - but Everyman Chess's "Starting Out" series is what you want to get to start out learning ideas like strategy and common tactics in your opening. A book like NCO is great for finding a repertoire you may enjoy, but I think you should be looking at specific books for learning to actually play the opening, not just learning the moves.
The limitation of a database for weaker players (myself included - no offense to anyone on this site!) is that often the moves that GMs don't play will not appear in the database, so you may have to work out why they aren't played. In contrast, any good opening book explains moves which are mistakes or blunders and guides you through.
I was under the impression that books like MCO and FCO are more than lists of moves -- that they provide analysis of what's going on as well. After reading a bit more on Amazon, I'm leaning toward FCO -- MCO is criticized as being full of typos, and FCO is praised: "For a beginner or intermediate, this book will give you enough theory to get started - your first half dozen moves with some deviations - but not enough to distract you from tactics and endgames."
I'm looking for a tool that will walk me through opening lines and point out the why as much as the what. Does FCO do that? Will an opening database do that as well?
It does exactly that, I have it and it explains every move thoroughly and explains why blah blah is bad etc..
MCO - List of variations
FCO - Explains moves behind moves.
John Watson has a set of books out call mastering the chess opening it is a 4 volume set, the first is on e4 the second on d4 the third is on c4 and the fourth is on irregular openings, I have the first three, they are great for understanding the openings and if that is what you are looking for I would recommend these books.
After reading the above posts, I'm leaning toward a book as opposed to a database, because I really want to consider the moves non-grandmasters make (hopefully, the discussion of why moves are mistakes will provide me advice on capatalizing on an opponent's bad move). I really don't have much of a repertoire at present, so I think I'll start with some general study and then move on from there.
The contenders I see are:
-- FCO
-- Everyman Chess's "Starting Out" series
-- John Watson's mastering the chess opening series
-- Nunn's Chess Openings
I'm going to read some more reviews and make a choice. Thanks to all for the thoughtful advice.
I'm finding myself in a similar position to the original OP in that I want a good opening book which also explains some ideas behind the moves/exploiting and avoiding bad moves. It would seem that FCO is therefore the book for me here. Can anybody confirm that this is a good idea (before I spent yet more money on chess books) or offer another suggestion?
I'm also a little put off in that I have come across a few people saying that databases are now the way to go for openings not books. So what about this?
I know that at my level I don't need to be too hung up on openings, just tactics, but at the same time I don't like being caught out early or getting into unfamilar territory by about move 5.
Thanks.
Any suggestions on the book?
Edit: I've ordered the book anyway, so I'll see when it comes.
FCO is the only opening book I have, and probably the only one I will need for a long time. It is great for explaining the ideas behind the moves and how to proceed from the standard positions reached. As you find lines that are interesting to you, you can simply research them a little further on a reputable website if you feel that it's needed. You made a great decision. 
I've been playing chess for many years and through luck and ability I do fairly well. I recently joined the USCF, and in my first tournament beat an 1800 and a 1500 before losing to a 1500. All my wins were on time (45 minutes per side), and in my loss I was crushed by an opening I didn't understand very well.
I've never done much study, and I'm looking at picking up Modern Chess Openings or Fundamental Chess Openings. Does either book offer any particular advantage?