I know its the obvious answer but try googling. The Hong Kong government even has a page in English about cycling in and around Hong Kong:
http://www.gov.hk/en/residents/culture/recreation/activities/cycling.htm
Work out what places you want to see/go (using guidebooks, tourist information websites, pinterest, Google Image search etc etc) to and then look for quiet roads between them. Don't use auto-routing. If there are any tour operators who go there look at there they go.
There's an awful lot of dross on Crazy Guy on a Bike (psmiffy's contributions are of course an exception). I would approach any online blogs with caution: you've no idea how good or bad the person writing the blog is at routeplanning or navigating. For all you know they have bypassed every interesting place along the route or taken the busy main road when they could have taken a quiet back road. I also suspect that there are bloggers out there who intentionally, or unintentionally, hype up the drama/difficulty of their tours.
There are also sites where that people an use to plan routes or show where they've ridden (eg Strava) these might be worth checking out, if only to give you an idea of where the locals go. But again be cautious as you have no idea whether the person who posted the route has even ridden it, or whether they are a clueless idiot or someone who really knows the area.
The Open Street Map maps for Hong Kong look pretty detailed to me and they are available not just online but to download to a smartphone, tablet or GPS. There's also of course Google and Bing and Here. It's definitely worth buying a traditional map but you can plan using digital maps if you need to.