Perhaps your reading skills were very strong for your rank (well, probably, if you started vol. 2 at 18k), but your shape/tesuji/whatever wasn't so strong... or something. I don't know.
I don't think it was particularly strong. All of my reading skill at the time was taught to me by that book as I was solving it. And vol 2 was quite hard back then for me, but I was a stubborn beginner and thought it would make me stronger to solve harder and harder problems. It would probably make you laugh to know how long I spent glaring at those problems and trying to visualize a third move on the diagramm. It wasn't at all about internalizing shapes for me at the time.
It's only recently that I realized the benefit of solving easier problems over and over again.
* Graded Go Problems Vols III, IV ($21 each)
Vol 4 is hard. It's really, frustratingly hard (for me at 15k). Vol 3 has mostly 1-3 moves problems. Vol 4 starts out with 5 moves problems. If you have like me serious trouble to read several branches five moves ahead, consider leaving that book for later. SoDesuNe advises it for SDK ranks, I think he's right.
* Mastering the Basics Series : 501 Opening Problems [Vol. I], 1001 Life-and-Death Problems [Vol. II], 501 Tesuji Problems [Vol. IV] ($25 each)
I just got 1001 Life and Death. It starts with one move problems, then gets harder and I think that's a good book at DDK. The opening and tesuji problems are said to be harder and not so useful before SDK. But I haven't read them.
* Get Strong at the Opening, Get Strong at Joseki (3 Vols. !), Get Strong at Invading, Get Strong at Tesuji, Get Strong at the Endgame ($21 each)
I have Get strong at Tesuji, I think it's a useful one to have at 15-14k and above. I don't know about the others.
Don't focus too much about getting a complete series, I think it's better to pick what you need at the moment and complete the series as your level goes up.