Documentation updates are being released quarterly by the BizTalk development group, and the April update is now available.  It doesn't install into a separate .chm file or anything like that, but it installs over a pre-existing install of BizTalk Server 2004.  So, the minimum install to read the docs is to install the Evaluation Edition of BizTalk Server 2004, but only install the documentation, and then install the documentation update.  Should provide some good weekend reading...  now, where's the Microsoft Reader version so I can load it on my iPaq?