First and foremost you should read the manual that ships with ToolBook itself (you can find a pdf copy here) and of course you should not ignore the inbuilt help file.
However there are a number of very useful resources, most readily available, that you may not have heard about. Listed below are as many as I can think of but if you know of others that really merit inclusion in this list then please let me know.
You'd be surprised at just how much information is secreted away in the ToolBook Knowledge base and it is routinely been added to. My own personal preference just to catch up on what is new is to have the search return results for articles added over the previous four days. This way all the latest articles are brought to my attention and of course there is always the option to go on and search for something specific. This link will take you to a preformatted search for the last four days worth of articles which you can always bookmark yourself.
Denny Dedmore of Sum Total Systems (who incidentally is also responsible for a lot of the articles you'll find in the Knowledge Base) wrote a couple of excellent books on ToolBook. Although written primarily for ToolBook version 8.5 they remain relevant for version 9.0 and the soon to be shipped 9.5. There is also the added advantage that they are now freely available from his web site in pdf format. You can find a link to them here but be sure to check out the rest of his site as there is a lot of useful information buried away within in portals.
If you're new to ToolBook, or even a seasoned veteran there's always something new that you can learn from the listserv. Hosted by Platte Canyon the listserv is probably the friendliest community support group I know and the generosity of the contributors never ceases to astound me. You can access the main listserv pages here but if you really want to make full use of it then make sure that you join up and get it fed directly to your mailbox.
Another great way to use the listserv is to Google it directly. Use this link to access a Google Search specially formulated to search the list.
Run by Simon Price, ToolBook.org takes RSS feeds from all the well know ToolBook sites and as such is a great one stop shop to catch up on all the latest news. You can find it here.
Peter Jackson's site, home of the freebee newsletter and a fantastic source of numerous tools that just make using ToolBook that little bit easier. If you've hit an impasse in a project that you're working on and you find yourself thinking there must be an easier way to do this then check out Peter's tools. The chances are there is an easier way and that the hard work's been done for you. You can find his site here.
As well as hosting the listserv, Platte Canyon produce a range of tools 'designed to improve the lives of training developers.' In addition to this they also host the annual TB Con which is widely regarded as the best ToolBook and E Learning Conference around.
No copy of ToolBook Instructor should be considered complete without a copy of this. Written by Tomas Lund, in one of those spare moments when not busy planning the next invasion of the English Fens, this little tool is indispensable. You'll find it here.