The standard engineering handbooks like Perry's Chemical Engineers Handbook and Machinery's Handbook will have sections discussing the different classes and styles of piping, flanges and fittings.
has quite a bit of info, plus links to other sites.
When I was starting out I found that supplier's catalogs were very useful. Ask in your buying office or your drawing office where they will most likely have a range of catalogs.
If you company is not big enough to have its own pipe standard manual check with the draftsmen in the D.O. - they tend to "accumulate" their own libraries as they move around from contract to contract.
And never stop asking questions. In my experience pipers are a friendly lot, always ready to help each other out.
BTW, ANSI (e.g. 125#FF flange includes old notations not actually correct. ANSI is a conglomerate organization that recognizes accepted standards. The flange standard is ASME. ASME Class 125 would be more accurate. Look for AWME B16.5 for flange information. The # or pound inserted is wrong. If you are a recent grad having progressed straight through your educatino, the pound references left the ASME standard long before you were born. You will see that explained in front of the standard.
Most people never read codes and standards. Lead, do not follow those people.