He pretty much single-handedly pioneered the abbreviated style of drawing, known as Ryakugashiki and produced a number of woodblock print art instruction books in that style. It was radical stuff in it's time, and personally I find it as fresh and contemporary today. It's possible to the seeds of illustration styles yet to be imagined like Art Deco and the graphic character stylings of the 1950's all pre-echoed in Keisai's loose yet essential linework. In fact even today as a 2D animator I can see direct comparisons between his work and the way we try to capture a pose with a strong line of action. Keisai would have made a fantastic animator.
What is particularly stunning about his work is that it pre-dates Hokusai's famous Manga sketchbooks by around 20 years. Although Hokusai's Manga are clearly magnificent, and if you can get hold of one of the modern reprint copies you'll be richly rewarded, in the most part, compared to Keisai they are quite traditionally drawn and fit within a distinct Ukiyo-e styling; unlike Keisai who seems to have completely gone his own way.
Above is an example from Hokusai's Ehon, one of a number of pages in this book where he moves away from his traditional style to an abbreviated style directly influenced by Keisai's drawings.
I always find it fascinating to see who influenced artists I admire, and Keisai certainly influenced Hokusai's more abbreviated work.
In Vincent Van Gogh's letters he mentions how he wishes he could capture the essence of a figure in just a couple of lines, like in the Japanese books and prints. He doesn't say which artist he's referring to but it would either be Keisai or someone influenced by him.
Have a look at The New York Public Library's digitised images of Kitao Keisai's 1797 book "Chôju ryakugashiki" Animals in the Abbreviated Style.
Other examples of Keisai's work are hard to find, but they are used to illustrate "The Silent Firefly", a collection of Japanese love poems translated by Eric Sackheim and published by Kodansha in 1963.