Essays: Straightforward: Just calculate wordage to the nearest 1000 words and this will equate to the number of hours.
Dissertations and Theses: These are a tad less straightforward, because of their length; the number of background verifications that may be needed; even the level of restructuring required. My recommendation would be the daily or weekly rate, rather than the hourly one, but do remember that you will receive regular updates on progress, and chats on the phone (which do not affect the price) are always very productive.
Help Guides and Project Briefs: These are probably best done on a daily rate.
Presentations, PowerPoints, Web Pages: These are priced on an hourly rate, with regular phone updates. It's also possible to set up some copy-writing, if that is also required
Technical Manuals: These will require a lengthy discussion, in order to assess the complexity and trialling required, so a weekly retainer can be agreed, dependant on the number of hours needed.
Campaign Projects: Depending on their scope and size, these are subject to any of the pricing options finally decided, after discussion.
Short Stories, Novels and Biographies: These are normally beyond the scope of the pricing options here, but again, terms can be agreed after discussion.
CV's and Letters: are subject to the specific rate that is applied to them.