I have just "redecorated" the bedrooms at La Casa -- splitting them into three groups and giving each group its own set of visions. Only one vision can be ended with entry into a locked room and only while exploring that room will one find the hidden book -- big hint on the word "hidden" (as in, if you can see the book right off, it isn't hidden, so keep looking).
I realize that by inviting folks to poke through someone else's room and that person's stuff, we are showing like zero respect for personal privacy at La Casa, but then after being in URU and reading all those diaries belonging to the DRC guys, I guess we shouldn't feel too badly. I covered the 'stranger intrusion" aspect with a quick call to the front desk and some reassurance about "sleep-linking" -- something that just happens.
Let me know if the hidden book is too hidden or too easy.