The choice of words here, I believe, depends on the tone (or the way) the sentence is conveyed. Namely, is it positive or negative? For example, are you having fun (e.g. you see it as a puzzle) trying to figure a 700 year old book, or do you only do it because you have to do it (e.g. you were assigned to) and you'd really rather do something else?
If the latter (negative) then most of the choices from previous answers would fit. Personally I would go with mentally exhausting cause I think that's the most common way to say it.
This 700 year old book is mentally exhausting!
If the former (positive) then I propose intellectually challenging (or mentally challenging)
(Warning: Do not confuse this with intellectually challenged which means the exact opposite!)
This 700 year old book is intellectually challenging!
Do note there is a fine line between the two because both exhausting and challenging imply difficulty of the task. Challenging however also appears to imply a reward for completing a task (in case of reading a math book it's the feeling of finally having grokked the material) while exhausting only focuses on its difficulty.