These books have little written Chinese in them and are therefore very easy to figure out. I'm still not sure why there are some sections with numbers and either "A" or "B" next to them - I don't think these correspond to the difficulty of the respective problem number because there aren't enough numbers (perhaps 75 "A" or "B" and 300 problems in the following section).
Each book has about 500 problems. The main problem section deals with Life and Death, the next section with Ko problems.
When I was perusing these books in Taiwan, I mistakenly looked at just the first few problems before deciding to only get volumes 5+6. I should have gotten the first four volumes as well as volume 5 reaches high kyu to dan level problems very quickly.
I would recommend 5 for 2k-2d, and 6 for 1d-3d (just estimates - I'm only 5-7k).
I only have the "answer" books (for teachers, or self study?). The books were 150NT each (about US$4.50).
These books may be similar to Graded Go Problems for Beginners, but I'm not sure (only have the first and second Graded Go Problems for Beginners books). I haven't checked if they're identical, but my guess would be no.