.icb is Infinity Chess's 'proprietary' book format.
the only .icb book i've ever seen is the default book that comes (installed) with the Infinity application (infinity.icb)
most Infinity players use Chessbase .ctg books with the Aquarium Book Adapter.
(the book adapter allows you use .ctg book outside of a Chessbase GUI)
there are detailed instructions available on the web on how to use the adapter