Shikantaza, sometimes called "just sitting," is the subtle and radically nondual practice at the heart of Soto Zen practice.
Because shikantaza is so easily misconstrued, at GBZC, we teach shikantaza only after someone has gained substantial familiarity with breath-counting. And because it's so easy to deceive ourselves with a story about "just sitting," it is absolutely -essential- to meet with a Zen teacher regularly in order to really plumb the depths of this powerful practice.
Coming to the Dogen discussion group is excellent preparation for doing shikantaza.
And the best two books to start with are Opening the Hand of Thought by Kosho Uchiyama and Realizing Genjokoan by Shohaku Okumura.
In our liturgy book, shikantaza is described in recitation pieces by Reb Anderson ("Guidance in the Absolute Practice of Shikantaza") and Shohaku Okumura ("Those Who Greatly Realize Delusion Are Buddhas").