You won't get dumber while thinking... 18-20 kyu

From a series of 8 pocket books with life and death problems, varying in level from 20 kyu until 3 kyu. Convenient size, easy to carry with you in the pocket of your coat.
2 problems per page, the answers are printed on the back on the page. Compiled by Mateusz Surma 1p, EGF professional.
Be aware that the difficulty of the problems is sometimes higher than the book suggests.