Last week I got tempted by a very big book. This one. Yes, I know, it is Lovecraft. But if you want to deconstruct Lovecraft you have to know a bit about what he wrote, and this book looks invaluable from that point of view.
Also it is ridiculously good value at only £25.
Of course I have dipped into it. You know that thing when you have written a story around an actual sequence of historical events, and then some new research revels that a key date you had was wrong, and it blows a hole in your plot… Yeah, that.