I recently went on a one-day retreat to spend some time in prayer and reflection, and during that time I read (most of) Ramon Llull's mystical reflections in his Book of the Lover and the Beloved, can be downloaded HERE. It is about 50 pages long and contains a short introductory essay from one of the foremost translators and scholars of Llull, E. Allison Peers.
Most of the meditations (366 of them) are on the nature of love and how the love of God is tied to grief and tribulation. From time to time he gets autobiographical and addresses missionary topic explicitly, in terms of how he has traveled far and wide to defend the honor of his beloved's name, and also his disappointment with the mediocrity of the average Christian of his medieval day.
But there is one verse that explicitly mentions Islam, and I wanted to share that with you:
The Lover (Llull) reproached Christian people, because in their letters they put not the name of his Beloved, Jesus Christ, to do Him the same honour that the Saracens do to the false prophet Mahomet, when they honour him by naming him before everything in their letters. v. 149
Presumably he is referring to a custom of writing the shahada at the beginning of any official communication, which does indeed name Muhammad.
You can find some other of my posts on Llull here, here, and here. A list of all the links is here.