This study examined 48 cadavers:

Kutoglu, T., Turut, M., Kocabiyik, N., Ozan, H. and Yildirim, M., 2012. Anatomical analysis of azygos vein system in human cadavers. Rom J Morphol Embryol, 53(4), pp.1051-6.

These also identify some variants:

Özbek, A., Dalcik, C., Colak, T. and Dalcik, H., 1999. Multiple variations of the azygos venous system. Surgical and Radiologic Anatomy, 21(1), pp.83-85.

Abrams, H.L., 1957. The Vertebral and Azygos Venous Systems, and Some Variations in Systemic Venous Return 1. Radiology, 69(4), pp.508-526.

Özdemir, B., Aldur, M. and Çelik, H., 2002. Multiple variations in the azygos venous system: a preaortic interazygos vein and the absence of hemiazygos vein. Surgical and radiologic anatomy, 24(1), pp.68-70.

Dudiak CM, Olson MC, Posniak HV. Abnormalities of the azygos system: CT evaluation. Semin Roentgenol 1989;24:47–55

Dudiak CM, Olson MC, Posniak HV. CT evaluation of congenital and acquired abnormalities of the azygos system. RadioGraphics 1991;11:233–246

This is a comprehensive review

The azygos vein pathway: an overview from anatomical

variations to pathological changes

Piciucchi S, Barone D, Sanna S, Dubini A, Goodman LR, Oboldi D, Bertocco M, Ciccotosto C, Gavelli G, Carloni A, Poletti V.

Insights Imaging. 2014 Oct;5(5):619-28.