Lots of patterns and sample projects. Unlike other blackwork books I've found (though I haven't found many), the history of blackwork is heavily covered. Drysdale focuses more on the actual patterns, which is great!
One of the best blackwork resources I've been able to find. Although it's over four decades old now, it has a lot of very useful material - not just a brief historical overview plus how-to with a wide selection of blackwork motifs and patterns, but some very astute advice on light vs dense patterns, designing your own patterns, etc.