Achmed The Mad
First, he really preferred to be known as Achmed the I Just Get These Headaches. He could become very disturbed, otherwise. Achmed was the great necromancer of Klatch, and when he could pull himself together, one of the major wizards of history. Stark, staring knurd from pots of Klatchian Coffee, Achmed aggravated his...symptoms while compiling the Necrotelicomnicon, one of the most dangerous books known to wizardry. His previous work : Achmed the I Just Get These Headaches's Book of Humorous Cat Stories, however, suggests a pre-existing mental brittleness.
The cynical and the ailurophobic, however, might be inclined to suggest that out of his two books, writing the Necrotelicomnicon actually came as a blessed relief after dealing with cats for his first book.
Perhaps Achmed's Book of Humourous Cat Stories is Pratchett's self-deprecating reference to his own work The Unadulterated Cat. It is also a possible reference to Old Possum's Book of Practical Cats by T.S. Eliot - a poet particularly disliked by Lovecraft (to the point where Lovecraft wrote a scathing parody of Eliot's work The Waste Land).