The user guide for the N900, in English and Spanish, is now available on the Nokia USA website. The guide offers a basic introduction to the operation of the hardware and software, and runs through the key functionality of the device. It's a good way of getting a handle of the capabilities of the device. The N900 should go on sale in the second half of October, although this will vary by geographic market.
As is typical for Nokia user guides, the level of detail is at only a moderate level. User guides are intended to introduce you to the functionality of the device and do not, typically, offer comprehensive coverage.
Extract from Nokia N900 user guide (source: Nokia)