FCO by Paul van der Sterren is probably a better choice (Fundamental Chess Openings). He'll sometimes take a paragraph to explain the rationale behind one move. MCO was a wonderful resource before we all had the internet and access to databases. I know there are some explanations in the latest editions, but simply not in the same league as FCO. Look up some reviews...
Any alternatives to MCO-15th edition?

I have a really worn copy of Modern Chess Openings from my high school days. I thought I'd order a more current version from Amazon, but it hasn't been updated since 2008 making it 12 years outdated now.
Are there any other alternatives? I do like having a physical encyclopedia if possible.
Not really. Theory is too deep to print such a thing. Once upon a time, a book on as specific opening was 144 pages and encyclopedias were 600 pages.
Now opening books are 400 to 600 per opening. For example, "Alekhine's Defense A Complete Guide", which would only be B02, B03, B04, and B05 in an encyclopedia, is now a 543 page book (written in 2018).
Encyclopedias are a thing of the past. Again, theory is too deep for that. Time to pick up a book for each opening you are interested in.

@ThrillerFan:
I think it depends on what you are looking for. If you are looking for the main ideas of each opening, FCO does a superb job. Rarely does the treatment of any line go deeper than 10 moves, but each step of the way is explained. At 479 pages, it's not exactly a superficial treatment, but of course you are correct that repertoire books are the go to for in depth analysis of an opening you plan to play. An encyclopedia still has a place though and FCO is considered by many to be the best. At my dismal level, repertoire books would just be overkill.

@ThrillerFan:
I think it depends on what you are looking for. If you are looking for the main ideas of each opening, FCO does a superb job. Rarely does the treatment of any line go deeper than 10 moves, but each step of the way is explained. At 479 pages, it's not exactly a superficial treatment, but of course you are correct that repertoire books are the go to for in depth analysis of an opening you plan to play. An encyclopedia still has a place though and FCO is considered by many to be the best. At my dismal level, repertoire books would just be overkill.
He also said the problem with MCO 15 is the fact that it is 12 years old. He was looking for something more recent.
If you want to use the technicality of FCO being 10 1/2 years old (Nov 2009), congrats.
But there is nothing of this nature written in the last 5 years, going on the basis of him looking for something more recent!

I have a really worn copy of Modern Chess Openings from my high school days. I thought I'd order a more current version from Amazon, but it hasn't been updated since 2008 making it 12 years outdated now.
Are there any other alternatives? I do like having a physical encyclopedia if possible.
You should get yourself Chessbase Chess Openings Encyclopedia 2020.

I have a copy of MCO-15 and I really love using it. As mentioned above the problem is that it's old, so I wouldn't study it on its own.
I like to look at the lines it suggests while studying inside of a database. I like it b/c it gives me a definite end point (only the moves in the table and annotations thereafter) so I don't get sucked into wasting time studying super obscure lines no one will play.
I also like seeing newer moves that MCO either doesn't mention or moves it dismisses. It's fun to try and figure out why certain moves are no longer in favor, or why certain new moves weren't seen before. It can be really instructive.
If you're just trying to figure out the spirit of the opening you want to play, FCO or a book focused on the particular opening you want to learn about will probably be better. If you want to dig into the theory on a particular variation, MCO can help give you some structure to your studies, but it should be coupled with an updated database.
@ThrillerFan:
I think it depends on what you are looking for. If you are looking for the main ideas of each opening, FCO does a superb job. Rarely does the treatment of any line go deeper than 10 moves, but each step of the way is explained. At 479 pages, it's not exactly a superficial treatment, but of course you are correct that repertoire books are the go to for in depth analysis of an opening you plan to play. An encyclopedia still has a place though and FCO is considered by many to be the best. At my dismal level, repertoire books would just be overkill.
He also said the problem with MCO 15 is the fact that it is 12 years old. He was looking for something more recent.
If you want to use the technicality of FCO being 10 1/2 years old (Nov 2009), congrats.
But there is nothing of this nature written in the last 5 years, going on the basis of him looking for something more recent!
Who cares. You could have a brilliant book on playing the opening written 100 years ago (Capablanca's Fundamentals of Chess was published in 1921).
Anything written after the year 2000 especially should be suitable all the way up to near the highest levels. The few updates to opening theory - if they even matter at your level - can be looked at and studied in your mind using chess databases. If you can't keep that material in your head you have no business going that deep in the first place.
The whole idea of updating chess materials year on year is marketing. While it may have made sense in the 60s or 70s it certainly doesn't today.
I have a really worn copy of Modern Chess Openings from my high school days. I thought I'd order a more current version from Amazon, but it hasn't been updated since 2008 making it 12 years outdated now.
Are there any other alternatives? I do like having a physical encyclopedia if possible.