This is a non-inclusive list of suggested Reading Material in no particular order. Then if you look at the references and footnotes in those books it will lead you to many more works. There are links to some current periodicals below theses books.
The Religions of Man by Houston Smith
The works of Idries Shah. "Learning How to Learn" is a good start.
The works of Samuel Lewis. "In the Garden" is a good start.
The works of Inayat Khan
The works of Philip Kapleau. The Three Pillars of Zen is a good start.
The Rubaiyat of Omar Khayyam
The writings of Swami Ramdas, (1884-1963)[Anandashram]
The works of Sri Auobindo
Zen Flesh, Zen Bones by Paul Reps
The Conference of the Birds by Farid ud-Din Attar
The Tale of the Reep Pipe by Massud Farzan
Seven Arrows by Hyemeyosts Storm
The works of Kahlil Gibran
The works of Confucious
Sufis of Andalusia by Ibn 'Arabi
The works of Rumi
The Walled Garden of Truth by Hakim Sanai
Stories of Jesus in the Koran
The gospel According to Islam by Ahmad Shafaat
The Secret Garden by Mahmud Shabistari
Journey to the Lord of Power by Muyhiddin Ibn 'Arabi
Following is a short and very incomplete list of some of the writings considered as "sacred texts" or source material of some of the major religions. There is benefit in reading many of these; however it is also full of difficulties. That is because of the nature of the translations. In many cases they no longer represent the work, sayings, or writings of the inspirer. They often tend not to carry forward many of inner meanings. However, that does not mean that they should be disregarded. Much can be gained on many levels by working with these and other similar materials. Care should be given to attempt to attune, especially through the breath, to the intent of the writings and writer. Adequate time, preparation, and patience is needed to do so.
The Bagavad Gita
The Jap Ji
The Hebrew source writings
The Nag Hammadi Manuscripts
Links to some Periodicals that may be of interest: