Wikipedia correctly describes it as "the official martyrology of the Catholic Church. Its use is obligatory in matters regarding the Roman Rite Liturgy...it provides an extensive but not exhaustive list of the saints recognized by the Church."
It is a spiritually inspring read as a daily calendar recounting the stories of saints who gave thier lives for Christ. Now you can pay $40 for a paperback copy from Amazon, or you can download it as a part of the public domain for free here:
Thanks for the link. I just downloaded it. Now will I ever read it? Probably not regularly, but I will look them up when a martyr's memorial or feast comes around on the liturgical calendar.