Life In 19x19

Does anyone have recommendations for a book regarding the opening or early middle game that is suitable for a strong SDK or weak Dan player?

I have In the Beginning, which seems too simple for my level, and Get Strong at the Opening, which is alright I suppose, but I'd like to find one or two better books covering this aspect of the game, which I feel lags behind my late middlegame and endgame play at the moment.

I very much liked Graded Go Problems for Dan Players vol. 7: Opening and Middle Game Problems.
The opening problems are in majority multiple-choice problems (90 out of 127), but the middle game problems are not (except the first few ones).
The final problems ("7-Dan" sections, about 25 opening and 25 middle-game problems) are quite difficult.

In a different style, I also liked Catching Scent of Victory by O Rissei.
In the first chapter, he analyzes some real-game openings where he tried unusual moves / strategies (which seem influenced by Go Seigen).

If you haven't already read it, Kajiwara Takeo's The Direction of Play is also a favorite of mine.

There are many books that you could learn from. Attack and Defense by Ishida and Davies (Kiseido) is a good middle game book at the SDK level. The Middle Game of Go by Sakata (Kiseido?) Whole Board Thinking in Joseki by Yang. Yang's The Workshop Lectures vols. 1 to 6 (Slate and Shell) have a lot of material on opening and middle game play. A book on modern Korean style of go used to be available but maybe it is out of print now.