For detailed & in-depth looks into custom Glock handguns, I'd read
www.customizeyourglock.com or Robert Boatman's Living With book about Glock pistols.
The late Robert Boatman stated he didn't feel the need for most Glock semi-auto pistols to get trigger jobs.
A few custom shops & pistolsmiths are out there but for most purposes, a Glock can be factory stock.
Clyde
www.boatmanbooks.com