Although a wealth of rock encyclopedias exist and nearly all have a small section covering Mott the Hoople and Ian Hunter, there are very few books dedicated to them. It is impossible to do Mott's or Ian's careers justice in only a handful of paragraphs and, sadly, most of these encyclopedias contain errors (mostly minor, but one or two contain major "howlers").
Of course, there is Willard Manus' novel Mott The Hoople, which inspired the name, from 1966.
Then there is Ian's famous Diary Of A Rock 'n' Roll Star, chronicling Mott's 1972 American tour. Out of print for many years, it was (briefly) back in print in the late 1990's. Sadly, it is out of print again, although Ian now owns the rights to it. He's waiting for the right offer to come along, 'Watch this space', I guess...
In the 1990's and early 2000's there were the excellent Outsider and Just A Buzz fanzines, put out by long-time fans Sven Gusevik and Justin Purington respectively (Justin is now joint webmaster for Ian's official website).
In the mid 1990's there was Phil Cato's disappointing and entirely derivative book.
1998 saw Campbell Devine's Official Biography, which is the definitive work on the subject which gets an unqualified buy it recommendation from me.
From 1999 to 2011 there was the Mott The Hoople Appreciation Society. Sadly, the MTHAS is no more (and I'm not sure if back issues are still available).
Currently (2017) there's the massive coffee-table book We've Got A Great Future Behind Us and Overend's The Man Who Hated Walking about his walking exploits in 2003. In addition, there is (roadie) Philip John's You Rocked, We Rolled which chronicles Mott's final European tour with Mick Ronson in October 1974.