My Secret Unicorn: Stronger Than Magic By Linda Chapman

Lauren and Twilight are very busy using magic to help Lauren's school friends sort out their problems. But one day Twilight falls sick with a mysterious illness. Lauren is desperately worried but the vet can find nothing wrong. Soon Lauren realises that it must be something to do with Twilight in his unicorn-form, and she discovers that using his magic for trivial deeds has been sapping all of his strength. Then Lauren's little brother Max goes missing, and although Twilight is still very poorly he summons every last bit of his power to help Lauren to rescue Max from danger.