When writing macro libraries, must your library be saved under a special file type or can it just be a .asm? Also, how exactly does the include work: does it just insert the contents of the included file into the code, or does it only include the macros you use?

The file could be any ASCII text file and you can use any extension you want.
include directive works inserting textually the entire file. It is done by fasm preprocessor before interpreting it.

So if I put code into an include file and include it at the beginning of an ASM file, the code in the include will execute when I run the ASM file? This would be useful because I could set up the GUI in the included file, and it would be initialized automatically by the GUI macro library when I include it.

direcitve "include" will just insert text from included file into source file, and then wok with it as if it was single file. So if code would be executed if it was in single file, then it will be same when in two files, one inluded to another.

Thanks muchly! With your help, my HLL macro library is coming along very nicely.