Taiga spends a lot of her time over at his house, so much so that she could almost be considered a member of his family.And since Ryuuji spends a lot of time with Taiga, he opens up to her world and to a side of her that most people do not see.
Note that it can be applied to novels as we know them but also the typical light novels for teens and young adults, which are common in Japan and aside from being short include a number of illustrations.
Ookami-san and Taiga are both tsundere and both slowly crawl out of their thick shell of emotionlessness towards a boy. Lastly I hope i don't have to mention Kugumiya Rie's involvement in both series, do I? A daily-life/love story with a lot of humour and don't look like a harem type centring about adolescence and sentimental confusion.
Sort of similar characters also with a laconic guy who loves a cheerful girl, a small and noisy girl...
Taiga has a negative attitude towards others and will not hesitate to snap at people.
After a series of misunderstandings between Ryuuji and Taiga, Ryuuji is taken in by Taiga as her personal servant.