Rev Fr William Doyle, S.J was a saintly Irish Jesuit who wrote extensively regarding vocations in the early 20th century, he was appointed as the Military Chaplain of the 16th Irish Division and died whilst ministering to his flock during the battel of Ypres on August 16, 1917. You can read more about his extraordinary life here and a book on his life may be purchased here
I'll admit to being slightly bias as both 'Shall I be a priest' and 'Vocations' have been immensely helpful to me personally but I can honestly say that they are full of practical wisdom, sound theology and extensive references to Doctors of the Church and other preeminent theologians. I am clearly not alone in my opinion as both of the pamphlets sold tens of thousands of copies when they were published.
'Shall I be a priest?' can be accessed here
'Vocations' can be accessed here