I have a Droid X I plan to use in the same way, but unfortunately have little experience with it thus far to share. One app you might consider is a voice recorder; I forget the name but I found a nice freeware one on the app market and have already found this useful while hiking.
Just getting UTM coordinates isn't hard, I think a lot of freeware apps will do that; I don't recall which ones as I haven't been working with this aspect much (I'm carrying a standalone GPS on my upcoming trip).
Photos: the built-in software seems fine to me, I particularly like their panorama assist.
Google Sky Map: I'm not sure either, but it's pretty easy to test --- there's a tool out there called "Advanced Settings" that allows you to turn off the cell phone "radio" (while nominally still not putting you into "airplane mode"). Likley a useful tool in general.
You might also consider getting the "flashlight app". Obviously a hideous thing to use for battery life, but if it's dark and your normal flashlight is dead or lost ..
Books: I like the Aldiko book reader, you might have a look at that.
Battery power: two other options to consider are (a) carry one or more spare batteries, and (b) search the web for ideas on how to reduce power drain. There's a lot of stuff that a smartphone likes to do automagically that can suck power; with some study you can likely reduce that factor to get more out of your phone.