How to get 12V from (automotive) 7V to 42V?
You basically need a switched mode power supply that is capable of both stepping down and stepping up the input voltage to 12V because your input voltage range is greater than and less than the required output voltage. Therefore a boost converter will not suffice; neither will a buck converter.

The simplest power electronic converter that could do this is the buck-boost converter: http://en.wikipedia.org/wiki/Buck%E2%80%93boost_converter
This converter produces a negative output voltage with respect to the input voltage. However, if the output can be floating, then this is not a problem. Just use the negative output of the converter your ground rail.

If you can't work with a buck boost, the next best bet is the flyback. This is similar to a buck boost, but can produce a positive output voltage because it is an isolated converter.

Another option is the SEPIC converter. However, this is more complicated to design and control since it uses two inductors.

To summarize, I would look at these converters in the following order:


Buck-boost
Flyback
Sepic