It is a shame because I can develop modules. The trouble is that I do it for an internal intranet so I don't need to worry about localisation or DAL but I want to create some commercial modules so I need to read up on this. It is a shame that there is not one source for best practices. I am hoping the module development book will cover this.

I'm in that same boat.  The module developer's guide has some of this, and ripping through the source of other modules has helped as well.  A lot of the information out there is older, DNN 3.x or prior.  And a number of the modules have really sloppy source, hard to follow and undocumented or poorly documented.  Grab some of the simpler module sources, the Hello World tutorials or Michael Washington's stuff.  Most of those are good to build on.  Some of the core stuff is the most convoluted, it's been upgraded and expanded by several authors and may not follow best practices.  I've also found that working from the Starter Kit doesn't work as well as you would hope.

Of course, my code is worse than any out there and a lot sloppier, so I'm not necessarily the best to answer this.  :)

Jeff