The Dickens Companions series is intended to provide the most comprehensive annotation of the works of Dickens ever undertaken. Its nature is factual rather than critical, identifying allusions to current events and religious issues, and supplying information on topography, social customs, costume furniture, transportation, the illustrations to the novel and so on. Identifications are provided for allusions to plays, poems, songs, the Bible, the Book of Common Prayer and other literary sources.
I'm reading this alongside TTC, obviously. It was especially useful in the beginning, when the book is heavier than usual on historical references and allusions. Has some nice quotes from Carlyle, etc., on which Dickens apparently based a lot of the book.
It also has bits of the manuscript from where he changed things, and I guess I'm always interested in that as a writer.
The book occasionally does not provide information where I would like--definitions of domestic items, customs, which would not be familiar to a modern person--or when I find plot points a bit confusing. I suppose that's what Cliff's Notes are for. :P