Shouldn't the Poet say che m'ha fatto cercar IL tuo volume ? Or is it that at that time there was not definite distinction between il and lo ?
You are right to note that in contemporary Italian only il can be used before a consonant, and the ending of the preceding word is irrelevant. However the rules regarding the usage of il vs lo have changed in the intervening centuries. From Rohlfs Grammatica storica della lingua italiana e dei suoi dialetti, paragraph 414 (my translation)
In ancient times the usage of the various forms [of the article] was partially more free, and moreover there were some additional forms available. Compared to il, lo had a wider usage than today. Originally, the two forms were determined by the phonetics of the sentence. The first (even in the apheretic form 'l) was originally used only after a vowel ending, before a simple consonant, while lo was used after any ending and before any initial. [...]
Originally only lo could be used at the beginning of a sentence. [...] Even much later lo was often used after r [...]; usage that survived in the expressions per lo più, per lo meno, per la meglio.
Therefore, in the period when Dante was writing, one could not use il (or even 'l) after cercar, since the latter word ended with a consonant. Thus he used the form lo.