As with hiking books and flower books, there are many choices here and some of it is personal preference. I find the recently updated National Geographic Trails Illustrated maps cleaner and easier to read than, for example, the Tom Harisson maps (with the exception the super detailed 1:24K map of Yosemite Valley. That said, some people I know don't like shaded relief Trails Illustrated maps for reasons I don't understand. Generally speaking, these maps are the cleanest and most accurate to date. Since they used one of my photos for the cover of the SE quadrant map, I had some contact with the editors and I know for a fact that the map editors consulted with several backcountry rangers and went over these maps pretty carefully. Nevertheless, every map I've seen seems to get the trail mileages wrong from time to time and I'm not certain I trust some of these mileages either. I've compared the Ostrander Lake mileages on three maps, two trail guides and the trail signs, and none of them agree fully and, in this particular case, I think the mileages given by the Trails Illustrated maps, the lowest of the group, are probably off. But what's a couple of tenths of a mile between friends.
These maps come in two flavors: one 1:80K map for the whole park (graced by a photo of Nevada Fall taken by my amazing photographer friend Tuan) and a four-map 1:40K set, dividing the park into quadrants (the cover of the SE quadrant being graced by a photo taken by yours truly). The single overview map is adequate for most on-trail hiking. The four-map set is adequate for most cross-country hiking. They're constructed out of a tough waterproof plastic and they have the good sense to print on both sides, which makes them less bulky for a given amount of coverage. We have taken ours out times beyond count in all kinds of weather, winter and summer, and this still look almost new, so they're quite durable. I don't use a GPS, but the map says it has UTM grids and is "GPS compatible", whatever that means (how can a map not be GPS compatible?).
Trails Illustrated CD
Of course, you can also go high tech and get various electronic versions such as the Topo! California series, the Trails Illustrated CD version and the Garmin 1:24K GPS maps for all the national parks. First, let me say that the Trails Illustrated CD, which is sold in the park for $9.95 is an utter disappointment. In fact, I'm not even sure what it's official name is and only mention it to save you the $10. In my experience it works poorly (i.e crashes) and only has the less-detailed map that you get on the single 1:80K park map. In this case, I would just stick with the old-fashioned print version. I see no advantage and many disadvantages to the electronic version.
Garmin MapSource 1:24K Maps and the Topo! 1:24K Maps.
Now we're talking the real deal. These are genuine 1:24K Maps with lots of great interactive features built in and the ability to print custom maps. I have the Topo! one, but the Garmin one for all the western national parks looks quite nice too. You can search for geographical reference points, plot routes and then get elevation profiles and reasonably accurate distances. Also, you don't have to deal with routes that cross maps, as you can build custom maps and print as needed. The only real drawback is the price. These sets don't come that cheap. Garmin describes their set like this: "The new MapSource US TOPO 24K National Parks, West CD includes extremely detailed topographic maps of Eastern (sic!) U.S. National Parks that are ideal for hunters, hikers and outdoor enthusiasts. Plan your outing on your PC, then download maps and data to your compatible Garmin GPS to guide you on your next trek." Yeah, well, I hope their maps are more accurate than their descriptions on Amazon.com! In any case, I can vouch for the Topo! series without reservation and I suspect the Garmin maps are of similar quality. The question comes down to this — are you a California person or a National Parks person. That is, do you see yourself ranging up and down the Sierra, or going to Yosemite, Yellowstone and the Tetons? The Topo! maps include all of California (though I can't say I've ever mapped Fresno with it) and the Garmin ones include all the National Parks. Garmin also has a set on SD/Micro SD chip that covers all of California and Nevada. So I would decide based on geaography and also whether or not I had a Garmin GPS since, presumably the Garmin maps are guaranteed to be compatible (I do know that the Topo! maps let you upload waypoints, but at least with my GPS and map set, you can't upload maps).