There are some great ressources around, some I frequently relay on are:
The Buddha Net file Library on Zen, Chan
The Buddha Net e-book Library on Meditation
Zen Buddhism WWW Virtual Library
But my advise will be, look for a Zen Sangha and learn to practice by a good teacher.
EDIT:
Since you ask: Which readings would you suggest for the purpose of concentration meditation?, start with;
Wikipedia:
Dhyāna in Chan Buddhism and see also the footnotes,
Commercial, i.e. not free of charge:
Sit: Zen Teachings of Master Taisen Deshimaru
Zen Training: Methods and Philosophy
A Guide to Zen: Lessons from a Modern Master
Japanese Zen sites of different lineages:
From Rinzai-Obaku Zen
As expressed in the famous description of Zen attributed to Bodhidharma, “A special transmission outside the scriptures, not relying on words and letters / Direct pointing to the human mind, seeing one’s true nature and attaining Buddhahood,” the essential standpoint of the Zen school is not to depend on words and letters. Nevertheless, it does regard the study and chanting of sacred texts as an important part of Zen practice. The texts and sutras most often used in Zen temples are the following:
Links to Text
From Soto Zen
Basic Key Terms of Soto Zen Teaching
A great many people are now practicing zazen in the Japanese Soto Zen Buddhist tradition outside Japan. But, because the tradition is so new in other lands, their practice may not always firmly based on a good understanding of the teaching. Before we begin practice, we need some clarity about what we are doing and why we are doing it that way. It is important to begin the practice with an understanding of the basic principles and underlying teaching behind what we will be doing. We need accurate background knowledge and a good understanding of ideas about the nature of zazen so that we can practice properly and skillfully. We should not start blindly. Otherwise our practice would likely go astray and get lost.
Links to Text
On both sites find some how to's and or chapters about Practice.