Standard grammar and style handbooks
Widely used standard books fall into different categories: general, pocket guides, and discipline-specific guides. No handbook is definitive, as stylistic and even grammatical rules evolve. Ask your professors which guides they recommend for writers in your field(s), or consult with Charles Yarnoff, Ellen Wright, or Jeanne Herrick of the Writing Program to find out which guide might best help you.
Academic grammar sites
For questions about a specific document type or technical issue, the online versions of textbooks or online guides can be very useful.
Popular grammar sites