Consider Acoustic Cryptanalysis.
Keyboard Acoustic Side Channel Attacks are a good one... reconstructing likely keypresses from the noise of buttons being pressed on a keyboard. Anything capable of picking up audio frequencies will be just fine, and there's an entire decades old industry dedicated to snooping on people talking and literally any of those device will do. Modern microphones can be the size of a fingernail or a section of pencil lead. Things like laser microphones or particulate flow detection microphones are rather larger, but you can set them up remotely to observe a target, and could potentially work on electromagnetically shielded devices if they weren't also in a well soundproofed location.
You need to use machine learning to build up a model of the keyboard being used and the typing style of the target... changing devices and typists can confound this to some degree, but you can keep recording audio and the more you have the better you'll get at working out what people are typing.
This of course doesn't tell you what people are reading, but only the notes/responses they're typing. Consider though that you could create a separate computer worm with the intent of getting it transferred onto an air-gapped network or computer by a careless (or malicious) employee and an infected storage device (eg. USB stick) which would generate audio signals that were out of the range of human hearing, but detectable by your audio bugs.
But, I'm assuming you're going to say, what if there's no audio device on the PC?
Turns out electronic equipment is hella noisy, and people have done all sorts of clever things merely by listening to the audio-frequency noises emitted by various electronic components. Consider RSA Key Extraction via Low-Bandwidth Acoustic Cryptanalysis:
Many computers emit a high-pitched noise during operation, due to vibration in some of their electronic components. These acoustic emanations are more than a nuisance: they can convey information about the software running on the computer and, in particular, leak sensitive information about security-related computations. In a preliminary presentation, we have shown that different RSA keys induce different sound patterns, but it was not clear how to extract individual key bits. The main problem was the very low bandwidth of the acoustic side channel (under 20 kHz using common microphones, and a few hundred kHz using ultrasound microphones), many orders of magnitude below the GHz-scale clock rates of the attacked computers.
Here, we describe a new acoustic cryptanalysis key extraction attack, applicable to GnuPG's current implementation of RSA. The attack can extract full 4096-bit RSA decryption keys from laptop computers (of various models), within an hour, using the sound generated by the computer during the decryption of some chosen ciphertexts. We experimentally demonstrate that such attacks can be carried out, using either a plain mobile phone placed next to the computer, or a more sensitive microphone placed 4 meters away.
Beyond acoustics, we demonstrate that a similar low-bandwidth attack can be performed by measuring the electric potential of a computer chassis. A suitably-equipped attacker need merely touch the target computer with his bare hand, or get the required leakage information from the ground wires at the remote end of VGA, USB or Ethernet cables.
So there you have it. With 2014-era technology, you could do this trick with a mobile phone. The brains of a modern smartphone without the big screen (and without the screen the battery size can shrink right down) will do the job for you just fine, just so long as you can get it close enough.
Not interested in audio signals? There's plenty of electromagnetic options, but I don't go into all of them here.
I will raise van Eck phreaking as a classic example, which does work on LCD displays as well as old-school CRTs, a detail that some of the other posters here have missed. Its range is limited, and whilst the radio antennae required needn't be very large you do need a reasonable capable computer in order to do the decoding for you. Of course, everyone carries a "reasonably capable computer" in their pockets nowadays. The lower the screen resolution and the longer the cable connecting the signal source to the screen, the easier the attack. This means that eg. iPhones are rather less vulnerable than a decade old government desktop PC.