"Two biographical studies of Salt exist, the most reliable being George Hendrick's Henry Salt: Humanitarian Reformer and Man of Letters (1977). This work seeks to introduce Salt to new readers, a task which is unfortunately still necessary. Hendrick also reprints a number of unpublished letters written by and addressed to Salt. Stephen Winsten's Salt and His Circle (1951) is made and marred by its association with G.B. Shaw, who wrote a preface for it at age 95 (!) and provided other materials in remembrance of his friend. Winsten's penchant for imagined dialogue and undocumented conjecture makes one appreciate the obsessive answerability of the best modern biographies." — William Stroup of Keene State College
To the above we would simply add that the precious few bits of worthwhile information that can be gleaned from Salt and His Circle are picked up by George Hendrick in his Henry Salt.
There have been relatively few essays on Henry Salt, in part this could be due to the difficulty obtaining much of his writings. Hopefully as more material is published online this will be corrected.
No books have been published on the Humanitarian League, however a few excellent academic works have appeared including Dan Weinbren's thesis The Humanitarian League 1891-1919 and his dissertation Against All Cruelty: The Humanitarian League both of which provide much invaluable information. Gary K Jarvis 2009 dissertation The Road Not Taken: Humanitarian reform and the origins of animal rights in Britain and the United States, 1883-1919 is available via Proquest but we've not seen a copy so cannot comment on it.