I disagree with a previous comment made about DSLR users not needing filters. I want to spend more time taking photo's than I do sitting in front of a computer screen. I do enough of that with working in IT.
This is a superb reference on the types of filters available with example images of their effects and advice on how & when to use them. The book starts with practical advice on how to choose a filter system and choosing the right one means you need never have to replace your filters when changing your current camera.
You can see examples of filter effects from most filter manufacturers websites, but this book goes further giving examples of what they are useful for (e.g. which filter to use with tungsten light sources). Lee also suggests how you can make some of your own filters, how to use multiple filters and how to avoid vignetting (darkening at the image corners which can occur when using multiple filters).
This is my favourite of the 3 books I own by Lee Frost and I recommend it to anyone thinking of buying a set of filters. The section with the sunset and twilight filters was enough to convince me that I should buy some.