Who wrote The Guinea Pig Trilogy?

The guinea pig mother thinks she wrote it, and put her name on the front. Hazel thinks he wrote it, and is sulking fit to cause a thunderstorm. We think the guinea pig mother may be channelling a wilful Pig God, who is making her a lot of work. 

Is Pig Tails really the first guinea pig novel ever-ever-EVER?

Hazel thinks so, and that's what counts. We suggest any other answers be kept discreetly to yourselves.

It’s rumoured there will be four books in the Trilogy. Why?

a) Hazel and/or the guinea pig mother can't do arithmetic.
b) Hazel is a difficult muse and doesn't care for convention, or arithmetic.

Whose fault is it?

Overall, it's Hazel's fault. And it's the guinea pig mother's fault for letting a big guinea pig boss her about.
And it's Corrianne's fault. You can read about that here.