Available for free from Every Trail, the app offers some wonderful videos, podcasts, photos, and, of course, directions on hiking to the top of Half Dome. There are 18 audio cuts in the app, taking you from the trailhead at Happy Isles, across the Vernal Bridge, up the Mist Trail past Vernal Fall and Nevada Fall, on to the base of the "subdome," and finally climbing the cables up to the top.
The text in the app mirrors a lot of what Mr. Deutsch offers in his book: how to properly prepare for the hike; being sure to carry enough-- make that lots -- of water; the need for quality gear; even clothing recommendations. To make it easy, there's a checklist you can rely on to make sure you've got all the bases covered.
There's also a reminder that you need a permit to climb Half Dome these days. And since many folks come to Yosemite from far off, Mr. Deutsch even provides information on lodging options.
Getting down to the nitty gritty, he recommends that you start the hike before sunrise and set a "non-negotiable turnaround time" so you don't get stuck somewhere on the trail in the dark.
Reflecting some of the accidents that have occurred on Half Dome, he also urges you to maintain a watch on the weather and to turn around if "storm clouds are overhead, if you hear thunder, if it is precipitating, or if the ground is wet."
While the details of the guide are helpful, the best aspect of this app, at least from my perspective, are the videos that Mr. Deutsch produced to accompany the written material. In one he interviews Royal Robbins, the first man to climb the face of Half Dome, back in 1957; there's another wth Ryan Ghelfi, who ran the trail in a scant two hours and 30 minutes, and; there's even a video of Mr. Deutsch hiking the last 600 feet of the route, via the infamous cables, from the bottom up. (This segment gives you a good idea of how crowded with hikers the cables can get, and how steep the 45-degree angle really appears.)
Of course, the downside of the videos is that they take a lot of bandwidth and are slow to load on a smartphone and can really tax your batteries. Best to watch them at home. Finally, he recommends that you reload the app frequently, as from time to time he updates it with news about the park and Half Dome.