A very good practice book is Vaccai's method. It covers all the intervals, semitones, and then goes into technique.
Random tip for the thread starter. Are you trying to get higher or lower? Pay attention to your jaw when you are singing, that is, keep it low and free from tension. Practice the vowels individualy. A very good, and helpful exercise is to pick a single note, and do this:
Bra Bray Bree Bro Brooh
For the bra and bray keep your mouth about two fingers apart. For the bree it can be a little lower. For the Bro keep your lips rounded and slightly closed. For the Brooh keep your lips rounder than the bro, almost like a whistle. This is how your mouth posture should be anytime you sing these vowels. Make sure that when you do this exercise, you feel the notes coming from the same place. I cannot stress how much lessons will help. Try and find a good italian singing teacher, as the italians were right on point in singing method.
