Reading on Eswatini in English is limited but there are a few worthwhile starting points.
Travel Guides
The most practical current option is Bradt's Swaziland guide (most recent edition covers Eswatini), which is thorough on wildlife parks, hiking, and practical logistics. Lonely Planet covers Eswatini as part of South Africa, Lesotho & Swaziland, which is useful if you are combining your visit with the wider region.
History and Culture
Swaziland: A Political History by Alan R. Booth is a solid academic account of how the kingdom navigated colonialism and emerged as an independent state in 1968. It is dry in places but thorough.
Hilda Kuper's An African Aristocracy: Rank Among the Swazi (1947) is an older classic of social anthropology — still the foundational text for understanding Swazi royal culture, the Incwala ceremony, and the political role of the monarchy.
All the King's Animals: The Return of Endangered Wildlife to Swaziland by Cristina Kessler, with a foreword by King Mswati III, tells the story of the country's remarkable conservation efforts in the 1990s.
Fiction
Eswatini does not yet have a substantial tradition of English-language fiction set within the country, though writers from the wider Southern African literary tradition — Nadine Gordimer, J.M. Coetzee, Petina Gappah — provide useful context for the region.