I've been working my way through this book. I thoroughly recommend it! The pieces are cunningly selected to teach you things like using thoughtful fingering to get the best legato and sustain. It's very rewarding to go back to the easier pieces and practice your vibrato and dynamics.
Here are some recordings of my progress from Grade 4:
J. S. Bach, Largo BWV 1056: https://youtu.be/7ul9pVx1m7c
Fernando Sor, Opus 31, No. 2: https://youtu.be/wGKhhW_OCGk
G. F. Handel, Prelude: https://youtu.be/XZ3anN8jWZo