In point 3 of this answer RemcoGerlich implies that a good way to improve, for a class player, would be to develop the skill of “[taking] any position and [figuring] out what’s going on”.
It seems to me this is the kind of observation that has been made quite often before. Is there any kind of resource that provides a collection of positions for this purpose? Preferably ordered by difficulty and with some annotations to check your own analysis against.
Clearly, there are books about the middlegame but the ones that I know talk about various topics in isolation (providing positions which revolve solely around the topic of that chapter) and not about general analysis. (Not that I would like such a collection of positions to only include middlegame positions but this example comes the closest to what I have in mind.)
I am also not looking for tactical puzzles. There are plenty of books and apps that provide them. In case of a tactical puzzle it’s very clear what’s going on in the position: there’s an immediately winning tactic. (Or sometimes a single drawing move. But mostly it’s clearly one or the other.)
Annotated games could possibly be used for this, but it’s not trivial to select the positions that would be of interest for this without reading the annotations. Also, the strategical themes wouldn’t change so often during a single game so the amount of practice positions derived from an annotated game is quite low relative to the effort required to find them – clearly a selection by a renowned chess teacher would be far superior.
Ideally, I’m looking for a collection of positions that are decidedly not tactical and that have various strategical themes in them so the reader may analyse and evaluate the position, afterwards checking with the provided annotations. They don’t need to have a clear best move, it would be enough to just try and find all important themes and get a good sense of which side is better.