Very little is available in English that is both accurate and accessible. The website of the Esperantic Studies Foundation is a good start. The website has a series of essays by Asya Pereltsvaig which are excellent.

Generally in English I would recommend Robert Garvia's "Esperanto and its Rivals: The Struggle for an International Language". I also like the essay by Markus Krajewski "Organising a Global Idiom: Esperanto, Ido and the World Auxilary Movement before the First World War" in "Information Beyond Borders: International Cultural and Intellectual Exchange".

Probably the best single work in English on Zamenhof himself is "Zamenhof The Life, Works, and Ideas of the Author of Esperanto". Ian Richmond's very abridged translation of Aleksandr Korzhenkov's "Homarano: La vivo, verkoj kaj ideoj de d-ro L.L. Zamenhof".

While there are some historians writing in English on specific aspect of the history of Esperanto, like Bridget 'O Keefe, the best comprehensive works like Ulrich Lins' "La Danĝera Lingvo" are out of print in their English editions, Majorie Boulton's biography of Zamenhof is similarly only available in its Esperanto edition and second hand copies of the English edition are hard to come by.