I am guessing the book being referred to is 'The highly sensitive person by Elaine N. Aron'. I have it myself because I too am 'highly' sensitive. I did quite a bit of reading on sensitivity issues in the past. I guess from a spiritual perspective you could view what you are doing as empathising. You are absorbing another persons stuff and your body is trying to process and purge those issues for that other person. You're trying to help them out.

A good way to check if it is your stuff or someone else's stuff that is affecting you is to ask yourself questions. Is this my stuff? Is this someone else's stuff? Is this something else's stuff? you'll be able to feel a difference when you ask each question (you'll feel heavier or lighter). If it is your stuff you need to dig deeper and examine it and if it's the latter two you need to learn how to let it go/detach. Not be such a sponge. There is always a motivation behind our actions. It might not feel like it, but when you do take on someone else's stuff it is with consent... some part of you is choosing to take it on board for whatever reason e.g. it could be the need to alleviate someone else's pain or other co-dependence related issues...what I am saying is that there is reason why it affects you so when it doesn't have to. You can choose something different and you don't have to be 'stuck' with it.