Since Sigma designs has just recently allowed public access to some of the official Z wave documents, I would always start with those now, as @zcapr17 suggested. If you just Google you won’t find those right away because they are so new and so not as popular, but they’re going to be the most accurate. 
http://z-wave.sigmadesigns.com/design-z-wave/z-wave-public-specification/
There’s also the issue that if you use a custom device type handler, then it can no longer run locally. Many people prefer to use the stock device handlers so that if the SmartThings things cloud is unavailable the device can still be controlled with a smart lighting automation.
So while you certainly can change to a custom device Type handler if there are features that you need, you just need to be aware that you may be giving up local operation.