Quick start circuit simulation using LTSpice XVII
Circuit Simulation with SPICE OPUS
It is a program used in integrated circuit and board-level design to check the integrity of circuit designs and to predict circuit behavior. Unlike board-level designs composed of discrete parts, it is not practical to breadboard integrated circuits before manufacture. Further, the high costs of photolithographic masks and other manufacturing prerequisites make it essential to design the circuit to be as close to perfect as possible before the integrated circuit is first built. Simulating the circuit with SPICE is the industry-standard way to verify circuit operation at the transistor level before committing to manufacturing an integrated circuit. Board-level circuit designs can often be breadboarded for testing.
There are a lot of books that deals with Spice and thus ngspice and circuit simulation. The following list is neither complete nor representative of the available literature. Suggestions are welcome! This book gives a clear description of the numerical techniques and algorithms that are part of modern circuit simulators, with a focus on the most commonly used simulation modes: DC analysis and transient analysis. There is detailed coverage throughout of the mathematical and numerical techniques that are the basis for the various simulation topics, which facilitates a complete understanding of practical simulation techniques. Comments by HV: So go for it: write your own circuit simulator!
It seems that you're in Germany.
the longest ride book free download