Lucinda Green's were good, and I'm half way through William Fox-Pitt's and enjoying it - it's a very horsey read iykwim, light on gossip, heavy on horse trials LOL.
I seem to recall from days of yore Pat Smythe's was brilliant (you at the back -yes, you, the one who said 'Who the hell was he?' you can stay in at break for your cheek) And Marion Mould's about Stroller, if you're into antiques.
Lucinda Green wrote "Up up and Away" about Be Fair and "Four Square" about some other horses. You might have to hunt around for them - think they are out of print. I found them in a secondhand shop. Up up and Away is very good and there's that photo of her jumping the scariest fence ever at Kiev.
Another oldie - 'Ginny' -Virginia Holgate/Leng's autobiography is a good read. It is probably hard to get now though as it was published in the 1980's.