Suggestions On Purchasing Excellent Reading Problem Books from Amazon.jp

I'm looking to add a couple of more books to the list below. My criteria is that the books offer excellent practice in reading by way of problems. This means that I am looking only for problem books. Hopefully, people won't just list every run-of-the-mill problem book, but rather those which promote or represent excellence in some form or fashion. Lastly, I don't have a need for the classics before the early 20th century (such as "Gokyo Shumyo") as they can be found for free in various forms easily enough. The books that I currently plan on purchasing are:
碁の力を強くする本 <上> - The Book to Increase Your Fighting Strength at Go, Vol. 1
碁の力を強くする本 <下> - The Book to Increase Your Fighting Strength at Go, Vol. 2
手筋事典〈下〉- Segoe Tesuji Dictionary, Vol. 3
I'm looking at the recently reprinted "Maeda Tsumego" set, but haven't decided on it yet, as there may be better options.

So far. I have made five book orders from Amazon.jp in the last year. The shipping costs were as follows (i) $73.55 for 13 books; (ii) $77.79 for 15 books; (iii) $78.01 for 15 books; (iv) $47.10 for 6 books; and (v) $81.61 for 17 books. The more books you order at once the better the deal you get on the shipping (but it's still quite expensive in any event).