There are books on how to build altcoins but I don't see any in-depth guidance on how to build a gambling site.
It seems to be all trial and error.
If you know of any technique or site or forum I would appreciate hearing about it.
The building or creating of gambling sites are part of web development which means it is covered by programmers. You can't easily learn those, there are so many ways to create a web site and different techniques and you must take the whole course to understand it. You must consider all of the things that are related to web developing since it's a wide topic and there's a standard that should be followed. Building altcoins is quite easy than web developing because there is already a platform that is essential for creating altcoins.
basically nothing is easy if you don't know the knowledge, and it will feel easy if you have knowledge in it and have a strategy to make it. making a gambling sites not only about web development, but also many other things, so that the site is in demand, legal staff, marketing staff, big funds and others thing are also needed if you want to create a great gambling site