I own and have read both. "Understanding Exposure" is a good all-round beginner book, and "Understanding Shutter Speed" is quite similar, just focused on shutter speed (therefore less general). However, after you have read "Understanding Exposure", you will already be a bit further from the absolute beginner, and will therefore learn less from "Understanding Shutter Speed" then you did from "Understanding Exposure". Some parts of USS are covered in UE, but not all.
What I like about Petersons books are not the explanations, but rather the photographs and the ideas he gives you. As a beginner, I find that his photographs inspire me to experiment, and his writing usually gives a simple explanation as to how the picture was taken. Very beginner friendly.
However, if you feel you're ready to face more serious challenges and books, then you should maybe skip this one.