A student put the following question out on Tumblr recently:

I’ve played violin for about 5 years but only in the school orchestra. Our teachers were never very good so I’m still kind of awful at understanding how keys and scales work and all fit together ( this is embarrassing omg) so basically I was wondering if you can think of any sources (preferably online) that go over all of the basics so I can review and finally understand what it is I’m playing? Thank you so much 🙂

While there a numerous online resources for this (and others on Tumblr have mentioned a few, such as Teoria), if you want a paper book to work your way through, there are some good ones specifically for string instruments:

• All For Strings – Theory Workbook 1 for Violin by Gerald E Anderson and Robert S Frost and All For Strings Theory WorkBook 2: Violin, available for the classroom string instruments.
• A good general music theory workbook set is, Master Theory, by Charles S. Peters and Paul Yoder. There is:
  • Master Theory Book 1, Beginning Theory Workbook,
  • Master Theory Book 2, Intermediate Theory Workbook,
  • Master Theory Book 3, Advanced Theory Workbook.
• For learning to read music, there is I Can Read Music: A Note Reading Book for Violin Students (Volume 1)
  by Joanne Martin, published by Summy-Birchard Inc. (There is also a viola version available: I Can Read Music: For Viola Volume 1)

Either of the theory sets listed above will take you progressively through the basic steps and lead you to a good understanding of music theory. The note reading book is a good beginner book for reading music.