Among the basic texts of Jewish religious literature, the prayer book, or Siddur, occupies a modest place.
It lays no claim to original thought or lofty inspiration like the Bible, nor to the breadth and profundity of the Talmud, nor does it give us the mystic insights and spiritual exaltation of the Zohar.
Alongside these great and voluminous works, the Siddur seems to occupy a side niche.
These other works are constantly studied, elucidated, and pondered with deep interest by the eminent sages of each generation, who write learned commentaries that their students labor to comprehend.
While the little Siddur, the common prayer book so familiar to all, is used for prayer and then put aside until the next prayer service.
Yet no book is closer to the heart of the Jew than the Siddur, and none has had such a profound influence.
Though other books may be more highly valued and admired, none has been so well loved and so uniquely able to penetrate to the very depths of the soul as the Siddur.
–Rabbi Adin Steinsaltz
From A Guide to Jewish Prayer, p.3, by Rabbi Adin Steinsaltz