Key dimensions of Thomas Mann's writing and life are explored in this collection of specially commissioned essays. In addition to introductory chapters on all the main works of fiction and the essays and diaries, there are four chapters examining Mann's oeuvre in relation to major themes. A final chapter looks at the pitfalls of translating Mann into English. The essays are well supported by supplementary material including a chronology of the period and detailed guides to further reading.
I read this (nearly entirely) for my bachelor thesis, since there was lots of context and information in it, which was relevant to a certain degree for my research question.
A tip (which I sadly only discovered at the end of my study): If you have a novel and you are struggling with a research question, pick up a Cambridge Companion book of that author (for there is actually a whole series of these books for all nearly well-known classical authors). While reading I came up with more and more possibilities for research questions. I wished I knew that sooner, since it took me weeks to come up with a good one.