This is an important issue as it is one of the main tools of ahl ul Bidah to try to discredit "Wahhabi" Salafi Scholars like Shaykh Al-Albaani and others.
I asked this question to Sh. Ahmad Bazmool hafidhahullah.
He breifly replied that it is not necessary for one to have an ijazah in general from a scholar nor to have an ijazaah for hadeeth or a particular book. I asked the shaykh this question after he recited to us his sanad going back to shaykh ul Islam ibnu Taymiyyah (rahimahullah) for the book Muqadimah fi usool at Tafseer. It was amazing.
At any rate, he pointed out that having an ijaazah is not necessary because it is highly possible that one who has an ijaazah does not understand the material / issues as well as one who does not have an ijaazah.
Also he mentioned that sometimes an ijaazah can be a frivolous issue. He mentioned that at times a scholar may give a general ijaazah. This means he gives you permission to teach what he has wrote or taught you, but you are not one of his regular students, he does not know your level of accuracy in memorization or the likes, nor does he directly test your understanding of what he wrote or taught. He gave us himself as an example. He said right now I am giving you my sanaad and giving you an ijaazah in this book, but do I know how well all of you have understood? No. So it can be an issue with very little weight behind it.
End of paraphrasing.
This reminds me of a quote that someone once read out of a book of Fataawa from Shaykh Muqbil in which he gave anyone who has read his books or listened to his tapes a general ijaazah to teach from him. Also going a step further stating that anyone has has done so can call themselves one of his students.
At any rate, the ijaazah and the sanad has its firm place in Al-Islam that cannot be denied especially in relation to Quraan and hadeeth. And it is truly remarkable to hear the scholars refer to them when they teach. So I hope that someone will follow up with more words of clarification concerning the importance / need for ijaazah in the past and in our times. But what I took from the Shaykh was that the Ijaazah / sanad has its place but that it should not be overemphasized as some of the Sufis of the west do. After all, what good is a sanad to the prophet SalAllahu alayhe wa sellem for a hadeeth if you have a deviant understanding of Islam?
And Allah knows best.
Abul Hasan Yusuf Al-Nebraski
"The example of the one who studies his Religion without an isnaad is like the one who attempts to ascend a roof without a ladder."
Abdullah ibnul Mubarak Sharaf Ashaabul Hadeeth 1/15 From the article: Virtues of Hadeeth and its people. (troid.org)