Hi Neil, standards vary by country so please UK-based producers contribute -- but the bottom line is: your author is the author of a book and will act as an advisor who may open doors. The job of an associate producer is something quite different.
Also, you want to retain the freedom to add other advisors later on without even feeling awkward about it. You need to protect your editorial control. Bringing him aboard as a partner is liable to make this more difficult. If you feel like all you need for your film is his book, without a great deal of other research, you could make it a doc based on his book, but it seems to me this is not what you are striving for as it would potentially compromise your editorial control. Whatever you do -- make sure to have a contract that clearly defines his role (whether he ends up getting a fee or not).
In general, you want to be careful with promising credits that are not absolutely bog standard like camera or editor etc. Credits are ususally subject to the approval of the broadcaster. All international contracts I have handled say something along the lines that the distributor tries to ensure credits are shown by their buyers but they can't promise anything. I.e. if your broadcaster thinks your credits are too long or not in line with their credit standards they will demand the right to edit them at their sole discretion.