Croquettes used to get fair representation in cookbooks. Chicken or turkey croquettes were a standard, ham was pretty common, and then there were the starch-based croquettes - rice, pasta and so on. (Except that back then, pasta was called macaroni.)
The primary driver for making croquettes was using up leftovers, something older cookbooks understood to be an elemental aspect of homecooking. Today's cookbooks are aimed less at people who intend to produce most or all of their meals at home and more at people who indulge in a cooking hobby on the weekends, while buying readymade meals or eating out during the week. Why plan for leftovers?
But as the economy declines, we will be returning to leftovers, whether we like it or not. Of course, reheating is fine for lots of things - no need to fuss with a leftover soup or chili, that's just getting better with age. But some things lose their appeal on the second or third showing.
Making croquettes is easy and the end product is delicious. But everything has its drawbacks - frying your leftovers does not improve your meal's nutritional profile. Moderation is key, and a small serving of croquettes can make a decent meal in themselves with just a nice leafy green salad or bowl of light soup on the side. Have fruit for dessert, and you can afford a little pan-frying.
The basic method for making croquettes is to mix up a rather thick white sauce (simple form - butter, flour, milk or stock), and use that to bind together the chopped meat, vegetables, rice or what have you. Odds and ends can be mixed in if you like (a bit of ham, cheese, some fresh parsley, scallion, that little bit of corn that wasn't finished up at dinner) just chop everything fairly small. Chill the mix until cold, then form into small balls or patties or oblongs and roll in bread crumbs. Chill again for at least an hour, and then fry until lightly browned. I don't deep-fry, I just put about half an inch of olive oil (not extra-virgin) in a cast iron skillet, and have at it.
If your base material is already fairly moist, like a risotto, you don't actually need the white sauce. An egg may be enough binder. Do use the bread crumbs, though, which will keep things from sticking to the pan.