We have a fair sized library of narrative style math books. My kids have always tested way out there conceptually and I give credit to the math books they have read for fun. I also like the Zaccaro book, though we only have experience with the next level book, I have heard nothing but good reviews of the primary book.

Here are some books that DS might enjoy: http://www.bellaonline.com/articles/art28465.asp

and here is my review of Life of Fred: http://www.bellaonline.com/articles/art25135.asp

BTW, my kids have gone back and forth between Singapore and several other programs because they seem to do well with a little novelty. It's ok to play around with different programs and even take a break from formal studies for a time. I don't like to leave math out altogether, but there are many ways to incorporate it into the day without workbook or problem sets.

hth-