An Analog computer is a computer which runs due to the principle of an analogy. Furthermore, values in an analog computer are typically represented by continuos values, which are refered to as analog in contrast to number or digit based values in an digital or binary computer. Basic information about analog computers can be found at the wikipedia:Analog Computer.
Ressources on analog computing
User:Ulmann has written a few books on analog computing, such as https://www.amazon.de/Analog-Computer-Programming-Bernd-Ulmann/dp/1978201931