If anyone wonders why Takagi didn't "buy the book to share" I just had 2 theories:
1) Takagi knows that even if she bought the art book Noshikata is still too prideful to let her Share it with him.
2) as far as we know probably Takagi didn't actually had the money and it was just an excuse to tag along Nishikata just to pull a "welp, I guess I forgot my money~" at the bookstore
Also, maybe its just me, but as far as I remember Takagi-san actually doesn't like 100% unrequited love that much and it was just her trying to have something to talk about with Nishikata, so she genuinely doesn't have a reason to buy the art book even of she has the money, so maybe thats why she just bought the soda to cheer him up and then smoothly suggested to go to her house.