I need to get up to speed about developing a commerce site using MS-Commerce server 2009 and SharePoint 2007. I was wondering where are good resources to get started, to explain the concepts and design, etc.
Commerce Server 200X is one of the least documented server products I have seen from Microsoft. There is one book that was released in 2009 called Software Smith's Commerce Server 2007. To buy the book you must order it from the German Amazon website here.
(Don't know why but StackOverflow can't handle the url as a link so I am pasting it here as well: http://www.amazon.de/Software-Smiths-Commerce-Server-2007-Smith/dp/3837038564/ref=sr_1_3?ie=UTF8&s=books&qid=1242725175&sr=8-3 You will have to copy and paste it to make it work but it does work.)
Even though it is for Commerce Server 2007 the API should be completely the same as what you will use in Commerce Server 2009. The major difference was that they added SharePoint integration so that you had a complete eCommerce and CMS solution based on Microsoft products.
Some other resources that you can scour are linked to here by Max Ackbar. Max used to be big in the Commerce Server space but about six months ago moved on to other opportunities.
MSDN is also an option though the reading is dense and the nuggets of usefulness are hard earned by those that venture deep into that mine.
To be clear, if your expecting an easy path (like learing BizTalk =P) then be prepared for a rough going. I would also add though that the payoff is potentially worth it. There is no more fully featured enterprise level eCommerce solution that I have yet seen than Commerce Server.
P.S. Learning SharePoint should be a breeze comparatively. There are tons of great books, code projects, and codeplex source code out there to give you what you need.