There is no doubt that for those who are seriously interested in the events in Peking in 1900 nothing will beat Diana Preston's extraordinary account.
However, this book gives in my opinion an excellent overview of what happened not only in Peking itself, but also in other places, making it a very good account of how troops from so many different nations collaborated together, despite their natural tendency not to trust each other.
There are interesting maps which will delight any wargamer and a very good bibliography if you wish to read more on that topic.
As usual for Chandler's books, it is rather Anglo-centric and tends to glorify the British participation, when in fact it was the Russians, Japanese and Germans that fielded the largest contingents and indeed saw most of the fighting.