If you want to dry vent the shower trap arm, then the dry vent needs to come off vertically (at most 45 degrees from plumb) and stay vertical until at least 6" above the flood rim. The picture from that old book fails on that last requirement, as I think does your drawing if I'm interpreting it correctly.
However, under the IPC, you can use the lav to wet vent both the WC and the shower, and the lav can join the WC before the shower. So you can just wye your shower drain in to the WC/lav downstream of the wye where the WC/lav join. As long as the resulting shower trap arm (from the trap outlet to that wye) has a total fall of no more than 2" (one pipe diameter), while falling at last 1/4" per foot. Meaning the maximum length is 8', which is only possible with a perfect 1/4" per foot slope.
To reiterate, the shower and WC may not join each other upstream of the lav joining. Since the lav is the wet vent, the fixture drains need to join the lav one at a time.
If you still prefer to dry vent the shower drain, you have to route it under a wall so that your vent takeoff can be vertical and stay vertical.