Anonymous: These may be the best problem books for all kyu players available in English.
RobertJasiek: I disagree. Vol. 1 is so easy that everybody should become stronger before needing to read it, Vol. 2-4 are useful reading. This is by far not the same as "the best English problem books". The problems in GGPfB have mainly one feature: their solution is straightforward to find and in this respect they are easy. GGPfB gives the reader the feeling "oh, I can solve these problems, so I must be improving". This is deceptive because GGPfB avoids difficult reading, explaining theory of tactical reading and by far most go theory of the studied topics. Maybe this combined with the low price make GGPfB popular but possibly "the most popular English problem books" is not the same as "the best". For the best, it matters how much one learns, how important and relevant the contents is and (since we are speaking of problem books) how well one learns tactical reading. There are other English problem books that I rate much higher in these aspects, that do not make it too easy for the readers but that require the reader to learn much and more important knowledge and skills (related to problem solving).