Wohlfhart (Op.45) 60 studies is the one I'm working on. It's available free as a download - I'm hoping it's because the copyright is no longer applicable. Anyway, I bought the Schirmer? copy because it's not exactly expensive.http://imslp.org/wiki/60_Studies_for_the_V...fahrt,_Franz%29
It must be well thought of. If you go to U-tube there are a few teachers who use it: namely ProfessorV , allysonviolin and Bernard Chevalier has recorded all 60 studies (and the Kreutzer). If you can make them sound anything like he does you'll be close to genius level!
There is also a study guide to them here:http://www.wps.pwp.blueyonder.co.uk/wohlfahrt_op45.htm
The level obviously varies. Early studies I guess are grade 1 or 2 but you might find study 18 and 25 challenging, especially played at marked speed. Study 60 is up or near grade 7/8 ish.
I also have a copy of the Kreutzer. It's pretty much beyond my capabilities (Grade 3) although I dabble with Etude 4, which seems to be an important study for up-bow staccato. One that Heifetz always recommended. Kreutzer is usually employed after Wohlfhart (I think) and it certainly seems to be a level above.
Many people seem to like Neil Mackay's 'Position Changing for Violin'. I haven't seen it but then again I'm stuck in first position. I do have a copy of his 12 studies and think they are excellent. Unfortunately I think it is out of print.