Symbolism, Imagery, Allegory
"Grief is a house" (11.1), Lennie tells us in one of her poems. In another, she describes Toby as walking with her from "room to sorrowful room" (11.64) of that same house, making her feel less alone. Toward the end of the book, her poem describes "The architecture/of my sister's thinking/now phantom" (30.1). In other words, when it comes to grief, it goes from being a building she's stuck living inside of to something a little less concrete, something more elusive—"phantom."
The image of being trapped in a building, walking through endless sad rooms isn't exactly a happy one, but it's the reality Lennie is living—especially since she puts off packing up Bailey's things for so long, so she is literally reminded of Bailey every time she is in her room. When it comes to Toby, he's trapped in the same metaphorical space, so it's no wonder Lennie is so drawn to him—he can come into her house when nobody else really can.
Houses are used in the book in one other way that's interesting: when Lennie compares her house to Joe's in Chapter 18. While her house is filled with wooden furniture and overflowing objects, Joe's house is all windows and high ceilings. In other words, even when Lennie is inside with Joe, she's reminded of the sky. Now be sure to read up on the sky as a symbol elsewhere in this section.