Neil is right! You should be able to find a great deal on Todleben (the familiar spelling) in almost any good general history of the Crimean War. Probably one of the most extensive accounts of his work appears in A. W. Kinglake's "Invasion of the Crimea" (published in from 6 to 9 volumes). You should be able to find this work in most major libraries in the UK, and many University Libraries in the U.S.
Todleben, himself, wrote a book entitled 'Defense of Sevastopol', a small portion of which a friend (Michael Hargreave Mawson) has translated and sells as a small pamphlet called "The Battle of Balaklava". I suggest joining the Crimean War Discussion Group at Yahoo Groups. Mike is the moderator, and many of the members would know a great deal about Todleben.
Luck in your researches,
Lawrence W. Crider Author, In Search of the Light Brigade Editor, The War Correspondent: journal of the Crimean War Research Society