The book is available at Project Gutenberg (I have not checked for other online sources). There's also a free audiobook at LibriVox.
Table of Contents
I. Sun-Worship. The Sources of Hallowe'en
II. The Celts: Their Religion and Festivals
III. Samhain
IV. Pomona
V. The Coming of Christianity. All Saints'. All Souls'
VI. Origin and Character of Hallowe'en Omens
VII. Hallowe'en Beliefs and Customs in Ireland
VIII. Hallowe'en Beliefs And Customs in Scotland and the Hebrides
IX. Hallowe'en Beliefs and Customs in England and Man
X. Hallowe'en Beliefs and Customs in Wales
XI. Hallowe'en Beliefs and Customs in Brittany and France
XII. The Teutonic Religion. Witches
XIII. Walpurgis Night
XIV. More Hallowtide Beliefs and Customs
XV. Hallowe'en in America
