I haven't read Nicholson's book (so many books to read, so little time) but I did a bit of digging around in academic reviews on JSTOR and it sounds like it's really good. The book is relatively brief but well written by a respected academic. The one impression I definitely got from the reviews was that if you want a history of the Hospitallers that extends into the modern era this is the one to get. It sounds like she at least continues her history up to the era of Napoleon.

Jonathan Riley-Smith's The Knights Hospitaller in the Levant, c.1070-1309 would be the major history of the order. I couldn't dig up any reviews on it (it's pretty recent though) but Riley-Smith is one of the pre-eminent historians of the Crusades. As the title says, though, this book is focused pretty much exclusively on the main period of Crusading with none of the Hospitallers' modern history included. It's also very expensive.

Riley-Smith also wrote Hospitallers: The History of the Orders of St. John a few years back which is a much shorter history of the order. The big difference I saw between this work and Nicholson's is that Nicholson's includes some modern history of the order, has fewer nice illustrations, and is more focused on the people who joined the order than it is on the achievements of the order more generally.

There are also quite a few general histories of the military orders which include the Hospitallers. The impression I've gotten is that Alan Forey's The Military Orders: From the Twelfth to the Early Fourteenth Centuries is currently the best work on the subject, although it ends around the time of the Trial of the Templars so once again any modern history of the Hospitallers or other orders is neglected.