It is established that the awliya have spiritual communications and unveilings. (Al-Furqan, 52)
Among the fundamentals of the people of Ahl al-Sunna wa al-Jama`a is the belief in the miracles of the Awliya: Allah created supernatural acts through them in all aspects of life, revelations, power, and impressions. This is known of ancient nations in Surat al-Kahf and in other Quranic chapters and is known of the early men of this Ummah amongst the Sahabah and the Tabiun and amongst the rest of the generations of this Ummah. It will be with them to the Day of Resurrection. (Al-Wasitiya, 33)
The miracles of the awliya are absolutely true and correct, by the acceptance of all Muslim scholars and Ahl al-Sunna wa al-Jama`a Only the innovators such as the Mu`tazila and the Jahmiyya and their followers deny them. (Mukhtasar Fatawa Misriyya, 603)
What is considered as a miracle related to knowledge is that sometimes the servant might hear something that others do not hear and see something that others do not see, whether in a wakeful or sleeping state of vision. And he can know something that others cannot know, through revelation or inspiration, or the dawning of necessary knowledge upon him, or truthful piercing sight, and such is called kashf, mushahadat, mukashafat, and mukhatabat. (Majmu, 11:313)
Regarding the fana of the awliya from his Al-Furqan:
..The second category of fana’ is oblivion towards others, which is attained by many of those who follow the path, because their hearts are strongly attached to remembering, worshipping and loving Allah, and not noticing anyone or anything else. Nothing crosses their minds except thoughts of Allah.
If this kind of fana’ overwhelms a person, his focus on Allah becomes so intense that he is no longer aware of anything besides, neither his own existence nor the dhikr he is doing, and everything diminishes in his senses every created being, the person himself and everyone else, and there remains only the Lord. What is meant is that everything is diminished in the person’s perception, so that he no longer notices or remembers anything, and is quite oblivious to what is going on around him. If this feeling becomes so strong that the person becomes confused and can no longer make proper distinctions, he may think that he is his Beloved, as it was said that a man fell into the water, and the one who loved him himself in after him. The former said, “ I fell by accident: what made you fall?” and the later replied, “ I was so overwhelmed with love for you that I thought I was you.”
Regarding the highest ranking awliya:
They have immense knowledge and sound discretion, so they see things as they truly are, and they see that created things exist only by the command of Allah, and are controlled by Him, submitting to His will and turning to Him. This increases their faith and insight, and whatever they see of His creation supports and increases the sincerity and submission of their worship of Him alone, with no partners or associates.
the kind of fana’ experienced by the followers of the Prophets is the praiseworthy type, and the one who experiences it is one of those whom Allah praised by calling them His God-fearing awliya’, victorious party and triumphant troops.
Check out Majmu Al-Fatawa, volume 10, by Shaykh Ibn Taymiyyah for more stuff: