The Rebus novels are in real time, ie he ages as things go on (which caused problems when he got to retirement age) and they do refer back to previous books, albeit lightly, so I'd say start towards the beginning, if not necessarily the first (which I remember as being less good). You can get cheap omnibus editions too, first 3, second 3, bundled together, often for pennies second hand on Amazon.
― koogs, Monday, 4 January 2021 02:12 (three years ago) link