There are many commentaries on the siddur. However, most of them focus in detail on the words of the prayers.
I'm looking for a commentary which looks at the "big picture" of each prayer. That is, where each prayer came from, when it was added to the siddur, which communities say it, why it is placed at the point where it is, why it is omitted on certain days, and so on.
(It doesn't actually need to be a commentary per se; it can be in any format, stand-alone or attached to some other work, etc.)