Austin Frakt has been doing an excellent job of reviewing the literature on this subject. This is from an overview of his recent posts:

The proportion of hospitalizations leading to rapid readmissions is shocking…But what causes readmissions?…Is this the fault of hospitals? Evidence about the extent to which hospital readmissions are related to quality of care during the index admission is mixed. Readmissions may be related to some notions of hospital quality or safety but not others. They are also related to many other factors that are not amenable to modification by hospitals. Joynt and Jha offer the most compelling and complete case that variation in socioeconomic status and hospital resources play large roles in variation in readmission rates. Hospitals that lose resources due to high readmission rates may be the very ones that can least afford it. Quality may suffer for the most vulnerable populations, which is the opposite of the policy’s goal…

The ability to predict hospital readmissions is modest, and little work has been done on predicting potentially preventable hospitalizations. MedPAC’s estimate that 79% of readmissions are avoidable is almost surely way too high. Here’s an evidence-based rule of thumb: 20% of Medicare hospitalizations lead to readmissions, and 20% of those are avoidable So, of all Medicare hospitalizations, perhaps about 4% lead to avoidable readmissions. That’s not nothing, but I’ll bet you thought it was higher.

The entire post and previous posts are worth reading.