My sister got me a low-carb cookbook one year. It didn't offend me, as I was diagnosed with diabetes that year, and she had just gone through gestational diabetes at the same time. We had talked about ways to watch carbs and shared low-carb recipes, so it was a very appropriate gift.
I think there are circumstances in which weight loss/workout gifts can be appropriate. Without knowing your sister's history with coworker and what conversations they've had in the past, it's really hard to say.
You said that she gave it to you to read.. were you offended?
No I wasn't offended
She was doing dictation and I hadn't brought my laptop with me so to keep me busy she went through her bag and kind of tossed it over saying that it was one of her SS gifts and maybe there would be a baking recipe (she knows I love to bake & look at recipes). I don't think she had even opened it at that point as it was still in her work bag and she later picked it up and started to look through it from the beginning. She put it down almost immediately and only picked it up again when I pointed out that I had seen a recipe for an unusual dish that had been on the menu at the restaurant we had eaten at.
Honestly my sister has come across blatant rudeness and meanness about her weight, so something small like this probably wouldn't be too bad relative to those even if she was upset. The lesson I took from it and what others have posted is that I probably shouldn't give gifts about weight unless someone has specifically said they would appreciate something of the sort just in case it does hurt their feelings - especially when it is a co-worker.
I can see how it would be different as other posters pointed out when the person is open about it - I have had a coworker who did South Beach and often talked about how they always keep an eye out for new books of recipes. In that case, I'd think I'd be ok to gift a cookbook about it. But if coworker only mentioned that they were on the diet and said nothing else, I'd go for something else.