For a while I've been wanting to learn ERP. What I would like to do is set up a system, and then practice running a business doing things like generating invoices, raising purchase orders, producing monthly accounts, keeping track of fixed assets etc. Then look at enhancing the system further by adding custom code.

I know there are a variety of open source ERP systems around, but really I would prefer a widely used commercial system as having familiarity with such a system would be more marketable. So are there any such systems that have a free developer version available? I've looked around, but I've had trouble finding such a system.

Also I would like to find a good book or more likely books to read on ERP. Ideally I would be able to get an indepth explanation ERP from a business perspective, ie the accounting operations it supports, how to do all the regular accounting tasks, configuration, business operations best practices, accounting controls and so on. In addition I would like an IT perspective, ie setting up the system, developing forms and reports. Unfortunately the few books I've looked at have been really superficial and totally inadequate. They either fall into the beginning programmer camp, where they use the programming tools but concentrate on programming 101 topics like loops, flow control etc. Or they cover setting up the system with lots of screen shots, but little substance as to why things should be done a certain way.

I have a programming background but no real experience in implementing ERP systems. Also I have a reasonable accounting knowledge, and have used ERP systems in various jobs, but only to a very limited degree. So are there any ERP experts who can point me in the right direction? Thanks.