Suspicious diagrams depicting PGT of D flip flop
You can see that on the book's D flip flop, there's a ring before the \$CLK\$ input.

That denotes an inversion, and can be though of as a short way of writing an inverter. That notation can be found for example in a NAND gate. In fact, the inverter symbol iterself can be thought of as a buffer followed by an "inversion ring".







simulate this circuit &ndash; Schematic created using CircuitLab

You can then understand that the \$!TRANSFER\$ signal leading edge is the \$TRANSFER\$'s signal falling edge.