For field access I would recommend the Apple products or Android Products. The Apps can be set up so that you do not need web access remotely. I use the following apps at this time when I am in the field:
I also intend to get a few more.
I also have an ebook of the Princeton Field Guides: Mammals of North America
The cost is not too much for the apps and I recommend the apps more than I would the ebook. The ebook I purchased did not have the ability to search like the APPS do. There are other apps for mammals and birds beyond the two I have mentioned.
I personally prefer the Android platform, and have used the two apps on two different Android phones and a new Android quad core tablet. They work great.
The advantage to the apple apps is that they were developed there first, and Green Mountain has created some "regional" apps that include several different subjects of a state or region, versus one app that has just birds, or mammals. They are slowly developing them for Android, but they have all of the larger ones available.
The thing I like about the apps as well are the ability to play sounds. I have used the bird app to pull in birds for a closer look, but you need to think through the ethical discussions on that, and what is legal where you are. For instance, to do that in Yellowstone would be illegal.
Glad I was able to point you in a good direction.