The easiest way would be to use the CRM Outlook client (making sure to specify the Outlook for, for Emails in the options). The standard Outlook email form is used but adds several CRM related items, one of which being Attach Sales Literature. Selecting this allows you to select and attach existing literature.
If that isn't an option, adding a look up to Sales Literature and using a plugin to attach the files would probably be the next best way but would involve creating .NET code for your plugin.