If you spend a lot of time in a web publishing environment, it pays to be aware of all the little things that make writing and layouts work — and not work. The wrong line height or kerning/font combination can be disastrous to readability. You can, of course, read books on the subject, but a ready reference tool can be had in this Typography Manual app, which looks about as to-the-point as such things can get.
It does in fact comprise a short book, along with some handy tools for the printer or publisher like conversion tables and a font ruler. It doesn’t have an iPad form yet, unfortunately. $4 in the App Store.