Well, that'll do, I thought. Let's start at the beginning. Hmmm, that's odd....
You may be having difficulty reading the text on the left as it'll be a bit small. You may also be having difficulty reading the text on the right, but that'll be because it's in Welsh. Here's a larger version of the English text:
This is all very interesting, but the trouble is that the picture above isn't of the Pontcysyllte aqueduct. The Pontcysyllte aqueduct looks like this, and, as you can see, spans the Dee valley in glorious isolation without a railway viaduct next to it. I know this because I have been across it, in both directions, in a canal boat. This was on our canal boating holiday in April 2000 - here's a couple of pictures:
Both of these pictures show us travelling northwards across the aqueduct; neither of them features me, sadly, since I took them, but as a bonus you do get my mate Martyn doing the hilarious "falling off an aqueduct" pose.
The aqueduct pictured in the book is the Chirk aqueduct, a few miles further south. We went over this one a couple of times on the same trip, as previously mentioned here - note the (slightly higher) railway viaduct also featuring in the picture. The Chirk aqueduct crosses the River Ceiriog, a tributary of the Dee, which this page boldly claims to be the fastest-flowing river in Wales.
As proof-reading howlers and basic failures of research go this seems like a pretty major one to me. I don't know if the pilots who took the pictures were given specific instructions as to what to photograph, or whether they just flew around and snapped anything that looked interesting and relied on the book people to label it accurately. Someone dropped the ball here in a big way, anyway.