The categories of research items were defined by ResearchGate. It is not possible for users to create new ones. However, there is the category Books. See Add new > Published research > Books.

If you mean how to change the category of a research item, e.g. from Article to Book, see see "How do I edit my research item's details?" in https://help.researchgate.net/hc/en-us/articles/14293081125777-Reviewing-editing-and-featuring-your-research for instructions. But note that some categories, e.g. Preprint, cannot be changed.