This volume could be used for both flute sightreading and as preparation for the sightsinging part of higher grade aural tests. The melodies are obviously challanging even if they are based on sol-fa, and you will need a good pianist to help you. Overall though it's a great musical experience however you use them.
Difficulty level, roughly compared to ABRSM exam grades. 0 is beginner, 9 is advanced (beyond grade 8).