This book is a rich source of studies, mostly scales, arpeggios, and long tones that represent the author's 30 years of teaching experience.
This material was written for cello but would also be helpful for any bass clef instrument; doublestops aren't explored here and so trombonists and bassoonists can also use this text successfully. Suitable for beginners but contains material even advanced professionals will always need to practice.
Also consider the complete Klengel, which adds 288 technical scale etudes which would build reading ability and technique and are perfect for classical music.