The sling is fantastic, I use a moby wrap and have been using it since DS was 1 week old. If he is having a bad day and not sleeping on his own I just put him in there walk around a bit and he falls asleep.
Also do you wrap your DD for day time sleeps, may be worth a try if you don't. She may feel like she has the security of being cuddled if she is wraped.

Also you asked about books to read. I have just finished reading 'the science of parenting'. It is a fantastic book. It probably won't solve your sleep problems but will explain to you what is going on in that little body so you can understand your DD and her sleep better.

Hope it gets better for your soon.