For a thorough study of Seidhr and Galdr, combine scholarly historical works, primary Icelandic magical texts, and contemporary practitioner manuals to cover both historical context and practical techniques
Goodreads

Historical and Academic Texts
The Galdrabok: An Icelandic Grimoire by Stephen E. Flowers – A foundational primary source on late medieval Icelandic magic, including annotated translations of spells, sigils, and magical staves (galdrastafir), with contextual commentary on Norse religion and magical practice
Old Norse Religion in Long Term Perspectives edited by Anders Andrén – Academic essays analyzing religion, magic, and societal shifts in Old Norse society
The Viking Way: Magic and Mind in Late Iron Age Scandinavia by Neil Price – Explores shamanistic practices in Viking culture, Seidhr, trance, and psychological aspects of magic
Witchcraft and Magic in the Nordic Middle Ages – Scholarly work examining historical magical practices and the cultural context of Nordic sorcery
Shamanism in Norse Myth and Magic, Vol. 2 – Folklore-focused reference analyzing shamanic and trance-related magical practices among Norse populations

The Chicanery of Seidhr by Rig Svenson – Discusses historical vs. modern misunderstandings of Seidhr, including textual analysis and the Viking Age context

Contemporary Practitioner Guides on Seidhr
Seidhr Magic by Dean Kirkland – Practical reconstruction of Seidhr techniques, tools, and trance rituals based on sagas, archaeology, and shamanic traditions

Seidr: The Gate Is Open by Katie Gerrard – Offers group rituals, trance practices, the High Seat prophecy rite, and guidance for modern Seidhr practitioners using Eddaic and sagaliterature inspirations

On Contemporary Seidr by Kurt Hoogstraat – Provides practical approaches while engaging with historical practices for modern application

Norse Goddess Magic: Trancework, Mythology, and Ritual by Alice Karlsdóttir – Focuses on Freyja-centered Seidhr and trancework with modern ritual applications