When I adopted my Himalayan and got him home... he was so thin I couldn't figure out why he was still alive. I could feel 3 sides to each vertebra....scary. You wouldn't know he was thin for looking at him though. Well, since then, I make a conscious effort to do a hands on all over check of all my dogs and cats at least twice a week. I can't say I did that before adopting that kitty though.
You may want to take her in for bloodwork, just to make sure that it is because she's not eating enough, and not a medical problem, and to talk to your vet about the best way to get her back into good body condition.
There's a chart somewhere... not crazy about Purina but this info is good...similar to the chart my vet uses to classify weight on dogs:
Purina.Com | Dog | Caring | Understanding your Dog's Body Condition
I used to mix 2-3 different brands of kibble together and fed mine like that, that way if a company goes under or changes their formula, I had two backups that they were used to... also found they did eat better... less finicky.