It's hard to call anything "unbiased" when it deals with Rhodes, but some come closer than others. Robert I. Rotberg's The Founder: Cecil Rhodes and the Pursuit of Power is probably still ruler of the roost in terms of comprehensive biographies--it's thick, well documented, and accessible although a little old (1988).

One interesting book more recent than that is Paul Maylam's The Cult of Rhodes: Remembering an Imperialist in Africa (2005). Maylam's of an older generation of historians, but the legacy and reputation points of Rhodes are very salient in light of recent controversy in South Africa and elsewhere over him. It's also a useful book to read if you intend to then consume any of the period (1897-1920) biographies of him that are sometimes over the top, especially L. S. Jameson's lionization. Jameson is, of course, another story...