I can't recommend a book in the UK, but I did a lot of research on this as it's one of the many diseases they thought my son might have.
If your SIL is used to cooking at home (i.e. doesn't rely on packaged and processed foods a lot), then it will be an easier transition for her. Unfortunately, gluten shows up in places it has no business in when it comes to processed foods!
If she cooks from scratch, then the hardest thing to replace will be bread. If she's a big bread eater, that will probably become her holy grail.
IIRC, there is a good gluten-free bakery in the north somewhere that does mail order (I think I posted about it on here somewhere). I seem to recall The Guardian also ran a piece on the perfect gluten-free bread that had a recipe.
A lot of traditional British dishes are already gluten-free: cottage and shepherd's pie, bangers and mash, even fish and chips can be done with a corn-based coating. Gluten-free pasta is also pretty decent.
It's really the bread that's the hard part. My son was not impressed with any of the store-bought substitutes. Fortunately for us, we did not have to stay on the diet so I never really got involved in intensive bread-testing after that.
HTH a bit. I'll see if I can find any of my notes about cook books later today.
