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Pretty sure mom was reading that book because of recurring nightmares about her husband being in hospital (as seen in this episode) and it was her last resort to calming her mind.
As far as Tracy goes, I interpret her dream as having an inner struggle going on in some form. However, what's interesting is dark Tracy refers to normal Tracy as T, the only other character that I remember calling her T is the new character, Nora. So, what if Nora has the ability to enter dreams and mask who she is? Again, I'm not really into that theory, but maybe that's enough speculation for someone else to craft a more elaborate theory on Nora.
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What's up with the creepy little girl reflection in mom & Jenna nightmare??