I love Jilly Cooper. I can quote swathes of her books verbatim. To my mind, her best work represents peerless excellence in the field of popular fiction. Unfortunately I couldn't say that about `Jump!'.
I agree with other low-star reviews that the main problem is too many characters, too thinly sketched. Jilly used to be superb at breathing individual life into her extensive cast, giving them real personalities, so I could instantly imagine each one in my mind's eye. In `Jump!', it's like she's stopped bothering. I honestly couldn't tell the difference between Alan and Seth, Miss Painswick and Tilda, Trixie and Amber, Joey and Chris, Phoebe and Romy, the Major and Alban, etc, etc.
As a consequence, I have no idea why certain characters are attracted to each other. People strike up unlikely dalliances seemingly at random. They are ALL, even the "nicer" characters, eye-rollingly undiscerning (even by JC standards), switching crushes and affections every other page in a feeble attempt by the author not to make it too obvious (but it still is) who is going to finish up with who. She's strayed from the usual saucy bed-hopping into pure parody. I ended up not giving a monkey's about the eventual pairings.
The constant theme of young girls reciprocating the lust of much older men is laughably unrealistic in some cases, downright disturbing in others. For example, one character is 14 when introduced and from then on, every reference to her involves a leering older man eyeing her up as a potential sexual conquest, including her own uncle. She has no other function in the plot - it's a far cry from Perdita in `Polo', who also starts off as a brattish, sexually provocative 14 year old, but has SO much more to her character. Later on there's a repugnant non-consensual "foursome" scene, which is basically an under-age rape. It did make me feel very queasy and I'm amazed JC's editor didn't persuade her to axe it.
Also I'm sorry to say, lazy writing for much of the novel, too quick to fall back on plotlines and entire sentences pinched from previous books. The howling duplications could make a great pub quiz for JC fans: "Which previous Jilly Cooper novel did the following quote/situation appear in....?"
The plot does indeed stretch credulity to the limit, but that's par for the JC course and wouldn't have bothered me if the characters had been more engaging.
Good points were: The research, impeccable as always. A few brilliant flashes of dialogue and one-liners tucked away amongst the dross. Some evocative descriptions of landscapes and the atmosphere of the racecourse. Some satisfying come-uppances for most (but not all) of the many unpleasant characters.
If you're new to Jilly, I'd strongly advise you to order `Riders', `Rivals' or `Polo' instead (or even all three at once, they're sequential and very addictive!). If you're already a fan, it's as likely as not you'll be disappointed.
I did plough on to the end, but finished it more as a nod of respect to the marvellous writer she once was than out of any real interest in the characters or the story. Very sad.