Indeed there doesn’t seem to be a lot of books describing Alexander II’s assassination in English. Adding the following to Lisa’s excellent suggestions :
The only biography in English I have is by Edith Martha Almedingen : « The Emperor Alexander II; a study » published in 1962 and she gave his death only 2 pages.
In her book about the Romanov dynasty, Victoria Cowles (« The Romanovs » 1971) has a chapter about Alexander II. Some interesting pages about the Nihilists plots and previous attemps to kill him though. She also quotes from Grand Duke Alexander Mikhailovich’s memoirs ( « Once a Grand Duke » 1932) who recalls that they were out skating with his cousin Nicky (future Nicholas II) and his mother when they heard the fatal explosion. They rushed to the Winter Palace in time to see the Tsar die.
Some books I don’t have but which I have read a long time ago; alas I don’t recall if there’s anything interesting :
Stephen Graham’s biography : « Tsar of freedom, the life and reign of Alexander II » (latest edition 1968 );
Former French Ambassador Maurice Paleologue « Tragic romance of Alexander II of Russia » , 1927
(At the time of Alexander II he was in St. Petersburg as a young diplomat and attended his funeral).
The old Alexander III biography by Charles Lowe (I seem to recall an interesting chapter at the beginning). New edition 1972.
Books I have never seen but which may be interesting :
« Five sisters : women against the tsar : the memoirs of five young anarchist women of the 1870s » edited and translated by Barbara Alpern Engel & Clifford N. Rosenthal, 1987 (Library of Congress has it under « Alexander II – Assassination » heading so it should be relevant)
Alexandre Tarsaidzé’s « Katia : wife before God » 1970
Werner Eugen Mosser’s « Alexander II and the modernization of Russia » 1992
About the master mind of the plot : David Footman’s « Red prelude : the life of the Russian terrorist Zhelyabov » (latest edition 1979).
Which reminds me of Virginia Cowles’ book called « Russian dagger; cold war in the days of the czars » 1969. I have it but just can’t find in my library. I seem to recall that she has a chapter about Alexander II’s assassination plots.
You might find some useful insight in general books about 19th century Russian history, such as Edward Crankshaw « The shadow of the Winter Palace; the drift to Revolution, 1825-1917. »
I would also suggest you try to find some British or American publications (newspapers, magazines) from March/April 1881. Most librairies should have at least one or two on microfilm if not any originals. My experience is that contemporary accounts were full of interesting details.
I have a number of other biographies and memoirs relating to Alexander II but there in French or Russian. I also have a photocopy of a fascinating narrative by Prince Romuald Giedroyc’s (Gedroitz?) (« La mort d’Alexandre II – extraits des mémoires du Prince Romuald Giedroyc ») published in 1881 : the most detailed account I have ever seen! 136 pages about the tragic events that happened on March 1st/13 and the days that followed (until the funeral). Sad but fascinating nevertheless.