It's when you open them that they get really fun. The kittehs can play with the empty bag, and I can check out the contents.
This one happens to be Menu #19 Beef with Mushrooms from 2002 (the menus are improved yearly and apparently this wasn't a favorite since it is no longer produced). In addition to the main course, you also get a side of yellow and wild rice pilaf, crackers and strawberry jam, cocoa beverage powder, oatmeal cookies, a plastic spoon, a flameless heater (that's the green thingie under the spoon), and a packet with hot sauce, salt, pepper, chewing gum, moist towelette, coffee powder, creamer, sugar, matches, and toliet paper/napkin.
MREs have an official lifespan of anywhere from 1 month (say in the hot desert sun of Iraq) to 10+ years (say in the cool expanse of my basement), but some say as long as the individual components aren't damaged, the practical lifespan is a very, very long time. In fact, in Justin Cronin's upcoming THE PASSAGE (which I reviewed during Dystopian February), people are eating and enjoying MREs 100 years after manufacture (this is fictional of course).
Find out more about MREs, including people doing taste tests on 20+ year old MREs, at MREInfo.com. It's really a fascinating site!
Anyway, now you know what books are taking about when they mention troops eating MREs.