There are many, many books on Dogchen, the principle meditation of the Nyingma Lineage. Dzogchen is a meditation of looking directly at one's mind and is similar to Mahamudra. Zen is, of course, a major type of meditation that originated in Japan and this meditation too involves looking at one's mind. 

In the early history of Buddhism in Tibet there was the famous debate between Kamalashila who was defending the meditation practice of gradually going though gradually improving states of mind and Hashan Mahayana who campioned a stance similar to Zen which was that one could suddenly or instanteously reach the highest realization. 

So in this short book of only 40 pages, Namkhai Norbu explains the Nyingma practice of Dzogchen and later on describes this encounter between these two forms of meditation.