Some studies that don't directly talk about how much of the prey item is absorbed by the snake, but do talk about the effect of digestion on ball pythons:
"Patterns of blood flow during the postprandial response in ball pythons, Python regius" by J. Matthias Starck and Christian Wimmer
"Functional Morphology and Patterns of Blood Flow in the Heart of Python regius" by J. Matthias Starck
"Digesting pythons quickly oxidize the proteins in their meals and save the lipids for later" by Marshall D. McCue, R. Marena Guzman, Celeste A. Passement
Not really about what eating does to them, but does talk about how slow their metabolisms are:
"Resting metabolic rates in boid snakes: allometric relationships and temperature effects" by Mark A. Chappell and Tamir M. Ellis
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