[glend (Glen)]

Kevin, there are a number of online tutorials that show step by step how to bring out the often hidden detail in files. I assume you know how to use a program like DSS to stack your image files to build a final image to process. There are some cheap options when it comes to processing, i used an old free version of Photoshop for a couple of years, then upgraded to the most recent cloud based version of Photoshop CC. I guess because i started with it i find it the easiest of the processing apps to use. I find PI particulary obtuse, but many here swear by it.
In so far as your desciption is concerned, levels and curves ( in Photoshop) are some of the main tools, but you also access to others. The 'art' lies in the balance of the processes you employ, but it is easy to 'push' your data too far. You might find some of Ken Crawford's Photoshop tutorials to be informative.
Free, or near free, apps like the old Photoshop CS version, can gave a limited tool set or lack fine control. Programs like GIMP are favoured by some.