No, I am not talking about Taylor Steven's debut novel, The Informationist, but it is that very book that introduced me to the term and made me wonder if there was such a term or profession.
What is an informationist, then?
An informationist is someone with a strong library science background combined with specialized expertise in a particular topic area. Informationists are usually embedded with the professionals they support rather than working out of a library. It appears to be used primarily within the medical profession (see the Wikipedia definition of "informationist."). Another terms often used (or that I am more familiar with) is that of "embedded librarian). The reference librarians I know who work at the NASA/Goddard Space Flight Center library for example, are extremely knowledgeable about both the science and engineering aspects of what NASA does. They're not embedded at the project level, but they are capable of supporting specific projects on an as-needed basis.
Next question: Where does the informationist fit in with the knowledge manager? Are these complementary roles?