Delft University repository published Jia Guo's MS thesis having a nice overview of column-level ADC architectures. While the thesis concentrates on DLL design, the overview is of general interest and covers many recent advances:
"DLL Based Single Slope ADC For CMOS Image Sensor Column Readout"
Jia Guo MS Thesis
"DLL Based Single Slope ADC For CMOS Image Sensor Column Readout"
Jia Guo MS Thesis