SciFi/fantasy includes a cat which materialises in a prison cell.

[Judy]

I read this book about 20 years ago. All I can remember is that it takes place on a planet run by witches. Three friends are journeying by ship, when they are put in prison and a cat materialises ad dematerialises in their cell. The book was quite humorous and might be part of a series.

[Andria]

Maybe A Sudden Wild Magic

or one of the Witch World series by Andre Norton?

[Judy]

Thanks, but the book isn't by either Andre Norton or Diana Wynne Jones as I have all their books. It's more like a Terry Pratchett, or Robert Rankin.

[Sarah]

Maybe The Witches of Karres?

[stormhawk]

The Cat who Walks through Walls, RA Heinlein

[El]

The Cat Who Walks Through Walls for stormhawk's suggestion

[Judy]

All good suggestions but it's not any of them - I've reread them all before posting my original question. I'm pretty sure it's not a well-known author ( or wasn't 20 years ago) as I reread those regularly. I'll have to go through my collection again for the books that are hiding behind the front row! One of my difficulties is that I may have loaned it to someone, as I often do with books I've enjoyed, so it's possible it's no longer on my shelves.