Decorative SHINY!
Because it was getting too long for Twitter:
So if you take the jacket off a hardcover book, you will see that the spine of the book (and sometimes the front) has words stamped onto it. They are usually done in foil (gold or silver, sometimes a metallic color), but also matte pigments are very common. There are gloss pigments available, but historically they are tricky to apply, so most printers discourage their use.
(But obviously someone is using them, because they still exist and I have a big book o' samples with them.)
The foil (or pigment) is literally a roll of the material, on some sort of plasticky film. The printer creates a die (usually metal, though I've seen ceramic or engineered plastic) from a file supplied by the customer. The cases* of the books are run through a machine with the foil unspooling over them, and the die stamps down on the cases while heat is applied. This causes the foil to adhere to the book and off the plasticky film.
The process for putting foil on the front of a book isn't much different. The jackets or covers are run through a similar machine (but usually larger). Paperback covers are a lot of fun because they're printed several on one sheet of paper, so the machine to stamp has to deal with the layout. In the case of mass market ppks, the covers don't all face the same direction. I don't know if the machine has multiples of the die and does them all at once or if it moves them around.
Ppk covers (and often jackets) sometimes get embossing as well, which is when the letters are raised up. (There is also debossing in which they're pushed in, but that's less common.) The letters can be raised up rounded, or they can be beveled. There are a couple of different cross-sections available, but simple rounded is most common, followed by simple beveled. Embossing dies have two parts (one behind and in front of the cover).
If the letters are to be both embossed and foiled, and all the foil and embossing on the cover match exactly**, i.e. no unfoiled embossing, and no unembossed foil, then both effects can be done at the same time with a single die. This is a really nice effect because there are no registration problems: that is, the embossing and the foil match perfectly.
If you have a cover with foil lettering for the title, say, and then embossing for the art, you can't do a combo die. In this case the foil gets done first, and then the embossing is done later. The embossing has to be watched carefully to make sure it lines up well with the foil.
On embossing: For things like type, which are solid and which get embossed to a uniform height, the die is spat out easily by a computer.
But sometimes you see a book where they've embossed the art on the cover. This is actually very tricky and is done by hand, and is usually done in a hard metal such as brass (instead of lighter aluminum) so it will hold the details. The guys who carve these--by hand and eye!--are skilled artisans who have to apprentice for a long time. They're trying to recreate the art in reverse as sculpture in metal and if they slip, the whole thing has to be redone. Which is why you don't see this sort of thing very often.
(You might see simplified outlines embossed, which is a cheaper way to get a similar effect, but it's not as good. If the art is relatively simple that's one thing, but nothing looks cheaper than cover art of, say, a coin embossed as a simple circle without embossing the raised head/whatever on the coin.)
Other questions on special effects? Ask in the comments!
*case = hardcover. "cover" usually denotes paperback cover, or the front cover of a hardback's jacket.
**there are exceptions where they don't have to be exact-exact. Say the author's name is BIG AND FOILED AND EMBOSSED along the top, and the title is just embossed along the bottom. Depending on relative placement, you could use a combo die and just put foil along the part with the author's name.