Arduino main site Teaching Docs... a start point
Analog Pins explained well enough to code with.
The code technique has been the forum specialty for longer than I got here in 2011.
My favorite tutorials are on Nick Gammon's MCU blog site.
Menu to a WEALTH of HOW-TO.
Use of millis and the principle of non-blocking explined well.
Reading without blocking, introduces state machines.
Just how simultaneous do you need to analog read? Is 1/5th of a millisecond close enough or do cross or additive effects require real simultaneous reads? Because then it's use ATtinys to do the reading.