I would heartily recommend you do get a book, there is just way too much to cover.
I'll give you a few things.
Although yo2001 summarized the process quite succintly, I would offer a couple different things.
If your prior paint job is SHOT, i.e. blistered, peeling, crazed, cracked or so totally sunburnt or worn through that there isn't much there, then yes sand down past the paint at least to the primer, and feather that out. If you are going all the way down, rather than using 400 grit sandpaper (hand or D/A) I would recommend you use 220 grit with a Double Action Sander (D/A) and no, this isn't your wax buffer with sand paper attached.
Sand down smoothing the prior paint and primer. If you encounter or have dings, dents or other imperfections, that require body filler do those first. Finish the repair so that it's smooth to the touch, (hint: don't look at the repair, let your hand feel the surface)
If the car is now finished and ready to go to paint, either use non-sanding primer or primer sealer. If however the body is still in need of some final smoothing, due to small dents or dings, then use medium fill primer, and wet sand the whole car down.
Wet sanding uses that black sandpaper on gray paper, and of course, water. The process is straightforward, wet sand the body using water to help cut the primer and carry away the powder. As you sand, feel the body for any imperfections, if you find any, fill these areas with Red-Cap (thickened primer) that you smear on with a spatula or rubber squeegie, let dry and then sand smooth. For the wet sanding process I would use 400 grit wet/dry sand paper. If you use red cap, I would recommend you use 280 or 320 to get it to almost finished, then top coat that area with new primer and finish that with 400 grit.
A second coating of primer isn't necessary, unless you cut through the primer to the metal. If you want to be assured of a problem free finish, rather than using primer, use SEALER. This helps seal the underlayment from the finish coat. This typically is a non-sanding item (i.e. no sanding required if applied smoothly) as long as you top coat it within the period alloted (usually 24-48 hours).
Painting in a nutshell, is apply your first coat slightly thin, i.e. just barely fogged on, 10 - 15 minutes later, apply a proper double wet coat, a third coat is up to you.
Let the paint cure the minimum period required according to the paint you used if you wish to clear coat. However, in order to clear coat, you'll want to "cut" the surface of the paint with 800 or 1000 grit paper to give the clear coat something to adhere to.
Apply clear coat, and let cure. This usually gives you a very shiny durable finish, for the utmost in gloss, you need to let the car cure for as long as 4-6 weeks before you once again, sand the car down, this time with 1200 or 1600 grit, then using a power sander with a buffing wheel, rub it out with polishing compound, swirl remover and finally glaze.
Now you see why I recommend the book? There are probably 15 questions for each paragraph above. Good Luck.
FWIW