I liked some of his exercises in that book, but the Bach chorales are hard to sight read, if you're just at the stage where you've finished the earlier exercises in the book, because the tenor line goes back and forth between the RH and LH. It might be easier to use the 2 part inventions or simpler movements from the English and French Suites, or the Clementi Sonatinas. The advantage of the Chorales is that there are so very many of them.