I have a 2003 BMW 325i and I wanted to know where I could find good technical documentation on it. I presume there is a maintenance manual or a Climber guide. Being that this is the age of the internet are there any resources on online that you trust to be maintenance worthy?
The best BMW e46 manual that you will likely be able to purchase is the Bentley e46 technical guide. It's big, heavy, and expensive ($75 to $100) but it's well worth it. While you might be able to find information online about a particular repair procedure, but this manual covers everything in detail, with lots of diagrams and pictures. It's so much better than a Clymer or Haynes manual that I would not even consider those as alternative reference books. Maybe the factory BMW manual is better, but I have never actually seen one in person.
It's not a service manual, but you can get technical drawings and parts lists from realoem. But if you're planning on doing anything beyond changing your wiper blades, get the Bentley manual. Sure, it's expensive for a book, but if it saves even one hour of a BMW mechanic's labor costs, you'll still come out ahead.
The AllDataDIY site is generally considered to be an authoritative source for both manuals and additional repair information such as recalls and service bulletins, part numbers, and repair time estimates. AllData is used by many repair shops
It does require a subscription. It looks like the minimum price-point is now $26 for a year of access for one vehicle. They used to have a short-term option, I believe it was $5 for 3 days, but even at $26 I'd say it's easily worth it.
Although, now that I've written this, it looks like they don't have information on BMW (though they do have Audi, VW, Mercedes, Porsche, and many others).
You can also try several forums. A lot of the times on the dedicated sites, users create their own DIY guides ranging from basic repairs to more complicated builds.
Check out the stickied threads first, they usually have short cuts to the guides in there :)
Also try RealOEM.com, it's a great resource for parts diagrams and part numbers. http://www.realoem.com/bmw/select.do
Here is a Q&A Site specifically targeted at BMW owners.
A good general discussion forum is BimmerForums