In his fifty page introduction to his method, Bottesini emphasizes the importance of solfege and ear training in general.
The Etudes found in his method are actually small solfege exercises in all keys. Students who use this book wisely will never need another solfege book.
As all modern editions are for four strings bass, some places are altered making it difficult to sing.
 
The Etudes can be used in several ways. I follow with my students the routine below:
 
1.    Reading the notes in rhythm (not singing)
2.    Singing solfege
3.    Writing at least two versions of fingerings (for example, with and without open strings)
4.    Playing and singing simultaneously (always tuning the playing to match the singing)
5.    Transposing using various clefs
 
Here is the complete original version of the Bottesini Etudes in a digital version.