[NSW specific] Not many books or websites provide information on off-track walking. If they do, it's usually lighter on detail, because the expectation is that if you need more information, then you probably don’t have the experience to be there.
My website (
http://bushwalkingnsw.com) has a few walks which are off-track or have off-track sections, but the caveats above apply.
There is in fact a lot of information out there, but it's not curated for you. For example, most bushwalking club journals can be found in the State Library. They have lots of info but you'd need to invest a fair bit of reading time. There are also many websites with photos and trip reports, but the information in those may not be of much use to you until you have done some easier trips in the same area.
Generally speaking your best bet is to build up some experience by doing many of the on-track walks in an area, and using the experience to devise less tracked or off-track walks. Or join a club, and go walking with leaders who already have some of that experience.
Most of John Chapman's book references are well worth obtaining.