Confidential Secrets of Bhajan (pdf)

This is a wonderful collection of verses and instructions by Srila Bhaktivinoda Thakura. Srila Prabhupada writes I have seen Srila Bhaktivinoda Thakura doing bhajana day and night.

He knew how Srila Bhaktivinoda Thakura used to do bhajana, because his eyes were not like ours. Our eyes can only see the material world, but Srila Prabhupadas eyes were transcendental, and he could see the unmanifest, spiritual world as well.