Koans are part of the teaching technology of the Zen school--and koan introspection (working with koans with a koan teacher) is one of the most effective ways to truly see the oneness of form and emptiness, the absolute and the relative. Working with koans is something that can only happen with a teacher trained in doing that work.
If you're interested in beginning work with koans, ask a teacher in dokusan.
We use the Harada/Yasutani Soto reform of the Rinzai curriculum. Most people never complete the curriculum--and this is not even the slightest bit of a problem. People who do complete it generally do so in 10-15 years.
We work through koans in this order:
- the koan Mu
- the koans clarifying Mu
- a collection called "the miscellaneous koans"
- The Gateless Gate / Gateless Barrier
- The Blue Cliff Record
- The Book of Equanimity / Book of Serenity
- Keizan's Record of Transmitting the Light
- The Precept koans
- Tozan's Five Ranks
The best book on koans to start with is The Book of Mu: Essential Writings of Zen's Most Famous Koan.
But be advised: the essence of Mu can never be found in a book!