I'm always studying soft lighting techniques in hopes of refining my portraiture repertoire. The subtle differences between a simple softbox and a light bounced off a flat or through a silk are intriguing to me, because while each of these sources is "soft," they're both distinctly different. The type of "soft" employed can have a huge impact on the look and feel of a shot.
One of the key differences between diffusing a pinpoint source (which is what's happening inside a typical softbox or when you shine a bare bulb through a diffusion screen) and a broad one (such as a light that's bounced off of a white wall or a flat) is that the former maintains some of the specular qualities of the source, while the latter is a truly broad source that's totally diffused. Specular sources produce harder-edged shadows and less wraparound light, although diffusing them certainly softens these qualities. Ultimately, this subtle distinction is the reason why softboxes and windows—both diffuse sources—look different.
A book light is a type of lighting setup that starts with a specular source, turns it into a broad one, and then diffuses it even further. It looks great for anything you might be lighting, but it's especially perfect for portraiture. It gets its name from the arrangement of the diffusion and the broad source, which are positioned in such a way as to resemble an open book.
Working backwards from your subject, position diffusion (a silk in a frame, for instance) wherever you might normally position a softbox in relation to the subject. Forty-five degrees from the camera, or even a true sidelight position, are both a good place to start. Then, position a bounce (like a white v-flat or large foamcore reflector) behind the diffusion (away from the subject) and at an approximately 45-degree angle to the diffusion. This is the "open book" shape that gives the setup its name. The greater the distance between the diffusion and the bounce, the softer the light will be.
Lastly, at another 45-degree angle, position the light source—be it strobe or hot light—to reflect off of the bounce surface, taking care to minimize spill directly onto the silk and elsewhere on set. The effect is that the bounce turns the source into a broad, soft light—one that's further diffused by the diffusion silk and becomes an utterly beautiful source. It really is ideal for beauty lighting of portrait and fashion subjects, and best of all, it doesn't require expensive modifiers or specialized equipment. You could re-create the effect with a lamp, a white wall and a bed sheet. No matter how you build it, a book light definitely does create a beautifully soft light source.