Hi Stephen,
There is some guidance in the manuals, but probably more around the mechanics of setting up the mail and how it talks to the process. This can be found in LANDesk Service Desk Mail section of the Administrators guide. The main areas to be aware of are what you can do once the link between a process and an email address has been established.
From an inbound perspective, you can only add notes to an existing process from an email. The email subject field must have the keyword (such as Update) and the reference number in it otherwise one of two things will happen. You will either get no note added, or if you are allowing new processes to be created via email, you'll end up with a new process. The best mechanism that I have used to ensure this works, is to send the user an email when the process is logged which contains a <mailto> statement with the correct syntax. This way the user can just click on the link and add information into the main body of the mail.
When it comes to the outbound mail, there are a number of ways that you can communicate with the end user. You can design your process to generate email automatically at certain points in the process, for example, process creation and process completion, or you can add an ad-hoc mechanism to create one-off emails to the user. This will use the Add Reminder action which can be populated automatically or manually. Again, this just depends on what you'd like to do. I have created email templates relating to different types of message, so for example, one would be just a standard update email and another could be a request for more information.
In either case the content of the email comes in two forms. There is the text that appears in the body of the email and also the content of a crystal report that can be attached to the email. Clearly if you attach a crystal report you can include a large amount of information in the report which you are limited on doing in the body of the email. In the body of the email you are limited to the size of the field, but you can build a complex set of information using either value types, or if you're running 7.3.x, you can also build the text using calculations.
I hope this gives you some thoughts on what is possible. My personal guidance would be to not over complicate this and also if you're not already doing so consider whether using the portal to provide end users with visibility of their logged processes might actually be a better approach.
Andy