I Can Read Music
I Can Read Music is the note-reading book most recommended by Suzuki teachers and teacher trainers, and is well-known internationally.  It develops note-reading skills one step at a time, using a set of progressive exercises in easy-to-read large print.  The books provide many repetitions of each new concept.  I Can Read Music was designed for Suzuki-trained students, but is also being used in string other teaching contexts, such as reinforcing reading skills or introducing the alto clef to violinists who are beginning the viola.
After the student has had an initial introduction to music symbols, I Can Read Music can be used at home by the child and parent with periodic checkups by the teacher.  By the time they have played both volumes of I Can Read Music, students will have encountered many of the most important rhythmic and pitch concepts needed to be a strong sight reader, and frequently they will have learned the skill of seeing notes on a page and hearing them in their head.
The I Can Read Music books have been designed for the fingerboard geography unique to the violin, viola, and cello, and so are best used for individual students or groups on the same instrument rather than homogenous string class instruction.  
I Can Read Music Volume 1 presents rhythm and pitch on separate pages, so that students can focus on one concept at a time.  The music is in big notes, with a minimum of visual information, and spaced so that it it easy to see that a half note takes twice as long as a quarter note.  The pitch exercises are all in quarter notes and use basic finger patterns.  The rhythm exercises are all on open strings, and introduce note values from sixteenth notes to whole notes, using time signatures of 2/4, 3/4, 4/4, 5/4 and 6/8.
I Can Read Music Volume 2 combines rhythm and pitch, and introduces key signatures.  Most of the exercises are in the form of duets, with the parts of equal difficulty.  There are some surprises, such as when a melody starts in one part and suddenly skips to the other, thus assuring that the players are truly reading and not playing by ear.  The student learns more complex time signatures with half-note and eighth-note beats, and those in compound time.  I Can Read Music Volume 2 can be used either as a follow-up to Volume 1, or as a beginning note-reading book for older students.