ePubPack[1] to create the ePub, FlightCrew[2] to validate the ePub, Calibre and iBooks on the iPad to preview. For Kindle books, I use Mobipocket creator[3]. Kindlegen[4], the tool Amazon provides to convert ePub to Mobi didn't output consistent results so I stopped using it.

I'm by no means an expert, so there may be a better set of tools out there but these have worked well for me.

[1] http://sourceforge.net/projects/epubpack/

[2] http://code.google.com/p/flightcrew/

[3] http://www.mobipocket.com/en/downloadsoft/productdetailscrea...

[4] http://www.amazon.com/gp/feature.html?ie=UTF8&docId=1000...