Any of the books by Bob Eynon are good for learners. They are available in different genres - western, sci-fi, romantic, adventure and so forth, and have vocab lists in them. Look for titles such as Y Gwr o Pheonix, Y Blaned Ddur, Y Merch o Berlin, amongst others. I have about 6 of his books and enjoyed them. There is also a book of short stories called Eistedd ar Groen Ieti, which is fun.
Hwyl,
Stu