AC analysis is a frequency-domain analysis. phasor analysis is the underlying mathematical technique, it computes the small-signal sinusiodal steady-state response of the circuit. I think you can refer to Kenneth S. Kundert 's "The designer's guide to spice and spectre".
Another advice: if you just want to use the EDA tools for your design, you need not to step in so deeper. Simulation methods and algorithms are a huge work and there are a large group of people and companies worked for it incluing Sysnopsis, Cadence, Mentor, Agilent, Ansys, etc....