Fascinating. Do not know which book Tirek has, but judging by the illustration on the page, it means that it mentions magical capacity in different beings other than the centaurs. Probably a textbook on magical creatures, the text on the pages is a little different from what the ponies use, although the magic symbols remain the same. Since Tirek without Sendak’s intervention here suggests that he can take the magic from others, it must mean that my headcanon that the centaurs can assimilate magic from the surroundings and perhaps living creatures, is not so wrong. But as seen in “Fiendship is Magic” # 2, a unicorn is very different, and the technique of draining the magic from such a creature was obviously too advanced for Tirek.
 
Thus, he needs Sendak as the most experienced and strongest magic user next to himself among the centaurs. But now we are definitely not in canon territory.

@Setsugekka  
Dark magic maybe or just a normal basic Equestrian spell learning book

Makes me wonder what this book was about in general

Interesting that Tirek had a book like this laying around. He must have studied other magical creatures a lot.