PSP is quite capable gadget for that (good screen and CPU). And I'm keeping some survival books in my son's PSP just in case. Unfortunately you have to hack the PSP firmware in order to use any third party software (so called Home Brew software) with it, which includes mapping, reading, databases, and many other titles. And that in turn will make it incompatible with the new official (off the shelf) games and OS enhancements.

I believe that at the present time the most capable devices of this kind are Pocket PC (or Windows Mobile) devices. They have tons of free and very useful software applications available online far exceeding anything available to iPhone and Android phones to date. They are easy to pair with the regular Windows computers. And easy to program (must have - for me, if the need arises - for others).

Another option is PalmOS based PDA's (even those with B/W screens). They have a much better battery life and even more software applications available, but lack in features and computational power (except for the most advanced models like SONY TH-55E). However, as you probably know, PalmOS is almost dead now.

Personally, I'm using HTC HD2 (T-Mobile) for all of my needs now. The only features I miss are the infrared communications and serial port - to control older electronic devices. But I'm working on that (it's so advanced that I could emulate those old school ports functionality with easy device modifications).

However if you are new into it - I'd recommend to go for Android devices now. They will rule the world eventually (what means that the growing open source development community will provide you with tons of useful and free software very soon).