This book is indeed a good, quick overview for easy to moderate day hikes in Yosemite. If you are not familiar with the park, it is an excellent quick study on where to spend your time hiking. A few notes of caution: the distances listed can be misleading because the figures are generally for one-way in only, so you will need to double them to get the total distance of the hike in and out. Also, there are some errors in elevation gain. The Happy Isles to Vernal Falls hike (a beautiful one indeed), for example, is listed as a 400 foot elevation gain, but in fact, the hike is a 1,000 foot gain for the 1.6 miles up (and up is the word) trail. Lastly, the book could do a better job of explaining that many waterfalls and some smaller lakes (like Mirror Lake) are likely to be pretty dry by the fall.
If you are a dedicated, serious hiker who wants to scamper well over a dozen miles in a day, climbing 4,800 feet up to the top of Half Dome, then no, this is not your book. For most visitors, it will be a good guide.