I work with printed output every day (being a graphic designer and all) so here's a quick walk-through:
Originally Posted by Rawfa
If all else fails you can just use your max-quality file for print output, but it'll be easier to send out your files if you downsample them to the optimum pixel count for the printing process you'll use.
To figure out the optimum pixel count, you need to know the dot density of the printing device. For continuous-tone photographic printers this is usually 300 printer dots per inch. For the "digital presses" used for photo books like the ones you can order from Apple (which look very nice if you prep your files properly) you can use about 200-250 printer dots per inch. If you're having your book printed by a commercial printer on an offset press, ask the printer to tell you the "linescreen" (typically for a book this will be 133 to 150 lines per inch) and multiply that figure by 1.5 (technically you should multiply by the square root of 2, but 1.5 is easier and close enough.)
Once you know the print device's dot density, you multiply that by your printed page size to get the pixel count you need for a full-page photo. For example, if your dot density is 250 and you're printing a book with 9-by-6-inch pages, the largest image you'll need to make is 2,250 pixels x 1,500 pixels.
You'll also want apply a small amount of 'unsharp mask' or 'smart sharpen' to your resized images, to compensate for the sharpness loss that happens in the printing process. Don't overdo this; use just enough to be perceptible when viewing the image at actual pixel size on your monitor
File format and compression depend on your book printer. Usually, if you want to do a one-off book like your friend's, you'll be using special software supplied by the book printer; this will take care of saving the book layout in the correct format. If you're doing your own layout and having the book produced by a commercial printer, most can use PDF format with the "press-quality" output setting. If you're supplying your own photos for someone else to assemble into a book... well, purists like myself still prefer PSD or TIFF files, but most people will be happy (and you'll get good results) with JPEGs saved at about the 90% quality setting.
Warning: There are multiple ways of doing this, and this discussion may well attract people who will tell you that everything I've told you is wrong and that the way you should do it instead is __________ [fill in the blank.] I do this every day and get paid for it, so I think you should believe me... but many of the other ways will work too.