The Collaborative International Dictionary of English v.0.48:

Prayer \Prayer\ (?; 277), n. [OE. preiere, OF. preiere, F.
   pri[`e]re, fr. L. precarius obtained by prayer, fr. precari
   to pray. See Pray, v. i.]
   1. The act of praying, or of asking a favor; earnest request
      or entreaty; hence, a petition or memorial addressed to a
      court or a legislative body. "Their meek preyere."
      --Chaucer
      [1913 Webster]

   2. The act of addressing supplication to a divinity,
      especially to the true God; the offering of adoration,
      confession, supplication, and thanksgiving to the Supreme
      Being; as, public prayer; secret prayer.
      [1913 Webster]

            As he is famed for mildness, peace, and prayer.
                                                  --Shak.
      [1913 Webster]

   3. The form of words used in praying; a formula of
      supplication; an expressed petition; especially, a
      supplication addressed to God; as, a written or
      extemporaneous prayer; to repeat one's prayers.
      [1913 Webster]

            He made those excellent prayers which were published
            immediately after his death.          --Bp. Fell.
      [1913 Webster]

   Prayer book, a book containing devotional prayers.

   Prayer meeting, a meeting or gathering for prayer to God.
      [1913 Webster]

   Syn: Petition; orison; supplication; entreaty; suit.
        [1913 Webster]

WordNet (r) 3.0 (2006):

prayer book
    n 1: a book containing prayers [syn: prayer book,
         prayerbook]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "prayer book":
   Sefer Torah, Torah, Torah scroll, Virginal, best seller, book,
   bound book, breviary, canon, church book, classic, coloring book,
   definitive work, euchologion, euchology, farse, folio, formulary,
   great work, hardback, juvenile, juvenile book, lectionary,
   limp-cover book, litany, machzor, magnum opus, manual, missal,
   nonbook, notebook, novel, opus, opuscule, opusculum, ordinal,
   paperback, picture book, playbook, pocket book, pontifical,
   production, psalmbook, psalter, publication, ritual, rituale,
   rubric, serial, service book, siddur, sketchbook, soft-cover,
   songbook, standard work, storybook, title, tome, trade book,
   volume, work, writing