I'd simply start with Frederick Copleston's Aquinas: An Introduction to the Life and Work of the Great Medieval Thinker, (London, 1991). It's a Penguin book so is available in e-formats and very cheaply as a paperback.

I'd also recommend picking up a survey book to provide some wider context, Kevin Madigan's Medieval Christianity: A New History, (Yale, 2015) would be an excellent choice, but it is still somewhat on the pricey side at £25 - although it is a hardback and you should be able to find a cheaper second-hand copy.