I was inspired by reading this post on another blog, as I have a few candidates of my own for this. It's quite rare that I fail to finish a book, but I did with Joan Dicken's Jill and Prince the Pony, which is real grade A stinker. I doubt even as a pony-mad child if I could have struggled through the waves of boredom which the pedestrian plot and characterisation roused in me, but as an adult I just couldn't. I failed.
Others I struggle with are Judith M Berrisford's A Pony in the Family series. This is supposed to be an educational series, teaching children how to look after ponies and ride. The educational bits are triggered when the hapless younger sister gets something wrong, which is when her hideously sanctimonious elder sister sails in and patronises her until any realistically written character would have shoved the elder girl face first into the muck heap and made sure she stayed there for a while. But no, she takes it all. The whole think makes me gnash my teeth in frustration that I cannot be the one to issue retribution.
There are other books I don't particularly like (the Silver Brumby books after Silver Brumby Kingdom, the Three Jays, J M Berrisford's Jackies) but the ones above are the ones that really get me venting.