score:20
This is explained — as it should be — as a frequently used abbreviation, on page 559, at the start of section called "Notes":
Abbreviations of principal primary sources used in the notes: […]
IAA: Richards, D. S., ed. & trans., The Chronicle of Ibn al-Athir for the Crusading Period from Al-Kamil fi’l-Ta’rikh, Part 3: The Years 589–629/1193–1231: The Ayyubids after Saladin and the Mongol Menace (2008)
— Frank McLynn: "Genghis Khan: his conquests, his empire, his legacy", Da Capo Press: Boston, 2015. worldcat
Which would land you at:
The referenced bit for this specific endnote then seems to apply to the 'chapter'/entry in that book:
The Year 625 [1227-1228] — Account of dissension between Jalal al-Din and his brother