"The thing that I'm most interested in at the moment is the so-called Infinite Energy solution - the possibility of finding new ways of tapping into virtually limitless sources of energy."
- Arthur C. Clarke

See also this usage from their 1981 novel Oath of Fealty.

"Sinclair wants to do some outside work while we're dirtside." He took out his pocket computer and wrote quickly with the attached stylus. "Let's see, we're about one and a half billion kilometers from New Scotland, that's uh, make it a hundred hours to turnover. About two hundred hours' travel time, plus what we spend on Dagda...

Apparently, they also included some sort of wireless link, because (elsewhere in the novel) it says that, when the officers were off duty, they "could always be reached on their pocket computers."

Another early mention of a small "pocket computer" or note-taking device with some mathematical functionality built-in is the calculator pad from Foundation by Isaac Asimov.

Asimov also mentioned a "pocket-computer" in his 1975 story Point of View.

As far as I know, the first pocket computer sold as such was the TRS-80 PC-1 in 1980. It weighed 6.0 oz., had 1.5 kilobytes of RAM, was programmable in BASIC and cost $230.

(Radio Shack Pocket Computer PC-1)