The bellwether is, to my understanding, the sheep that leads the drift of the flock. As a metaphor, it's maybe something like the trendsetter, or the avant garde. Willis's novel is partly about trying to figure out about how the bellwether is selected. I think I'll avoid spoilers around this one, as at the very least, there is a very interesting hypothesis proposed about this.
However, we still have the word to deal with. It's easy to understand how the misspelling, which implies an underlying mistake in meaning, came about. People probably think it has something to do with weather forecasting, with being able to prognosticate. In fact, a bell wether is not such a dynamic creature. Here is a nice definition that I got from They Have a Word for It:
The word comes from the 13th century and first meant a wether (that is, a castrated male sheep) which wore a bell. Wether is Old English and dates from the 9th century. Bellwethers were noted for their docile nature and were used to lead flocks, especially to the slaughter. A curious feature of old sheep slaughter-houses was that the final run before the slaughter-pen had a side gate in the fence, known as a bellwether gate. Along comes the dopey bellwether down the sheep run, followed by trusting flock, then, at the last moment, wallop!, the shepherd slips the bellwether through the bellwether gate and the other sheep trot on, oblivious to their imminent doom. The bellwether was then introduced to a new flock and the sinister cycle was repeated.
Okay, not exactly a character to emulate. On the other hand, well, just look at the picture and decide for yourself whether (no, not wether) there might be some compensations...