This is a before picture of my personal copy of a 19th century edition of the poetical works of Byron.
Practicing on a book where a mistake won't be catastrophic is always a good idea, and since this is a "not for sale" book, I didn't mind using it for experimentation.
As you probably know, most decorative gilding on books was done by applying gold leaf to a stamped or debossed area with an adhesive. (Restoration companies still use these techniques today.) This is one reason why gilding can flake off over time.
One way to restore the gilt on books is to do it similarly to the way it was put on originally. The idea of applying gold leaf might be scary, but craft stores these days have inexpensive leafing kits (roughly $7) that are remarkably easy to use. They come with an adhesive and roll of foil (gold or silver) and full instructions. While it helps to have a steady hand, you'll likely be applying the adhesive into a depressed area and the raised border will make it easier to "color inside the lines." You can use a very thin, tapered brush, a slender stylus, or even a toothpick to apply a small amount of adhesive to the area you want to gild. (I put one drop on a piece of paper and dip from there.) It'll be opaque white when you apply it, but will dry clear. At that point, take a piece of the foil, place it dull side down against the adhesive, and press with your finger. When you remove the foil film, the gold leaf will have adhered to your book. The affect can be stunning, but if the gold you're replacing is very old, it may be startlingly different from the leaf you're applying. I suggest doing a test piece on a separate sheet of paper and comparing them. Gold leaf does come in various shades of gold, so you may need to experiment.
If gold leafing still sounds too complicated, there is an excellent product called "Liquid Leaf" (about $3.50) that also comes in several gold shades and may actually match an old book's gilding better than foil leaf.
I tried several methods for applying the paint including a dip pen with a very fine nib and a toothpick, but nothing does as good a job as a very thin brush with a tapered tip. The bristles allow you to move easily to the edges with surprisingly good control. Just be careful not to load the brush with too much paint or you'll get a blob instead of a fine line. Also, always make sure you shake the paint well before you start. Gold pigments are heavy and thus notorious for separating and settling. If you're touching up straight lines, hobby shops carry very thin masking tape that you can use to protect the area you don't want to paint.
The after picture of my test book shows you examples of how both the gold leaf (flowers and wreath) and the gold paint (the letters in Byron's name) turned out. Was the change in the book worth the effort? You be the judge.