The 2 transistors in series provide a higher output resistance than just one of such a current mirror.
The gate connections of M11 & M12 are responsible for a relative low voltage "consumption" (≈ Vth+Vdssat) of the current mirror, that's why it is called a headroom saver.
In the case of your Figure 3.10 this connection additionally provides voltage-to-current feedback (similar to a CMFB circuit).
Vcmref via M13 and the diode-connected M14 supplies the necessary gate voltage for M9 & M10 .