Rabbi, could you please recommend a good and comprehensive book on the philosophy of mathematics? Or even a book that discusses it a bit?

By the way, with the questioner's pardon, I don't know if this is the place, but what does the Rabbi think about the claim that mathematics is not a discovery?

"Many object to this approach, because clearly the ‘discovery’ of the proof of Fermat’s Last Theorem is not similar to the discovery of an island in the ocean or the discovery of a plant that was previously unknown. The proof of Fermat’s Last Theorem involves a great deal of creative work, and to claim that it already existed and merely had to be discovered is not far from claiming that a new poem is not the poet’s creation but rather the discovery of the poem in the ‘sea of all verbal strings.’ According to this approach, all of mathematics is a creation of the human mind, and does not exist without it."

Haim,
there are two volumes by Shabbetai Unguru in the Open University series.

I don’t know where you’re quoting from, but the arguments here do not address the question of whether it is a discovery or not. The question of whether it is a discovery depends on the question of Platonism: whether mathematics exists before it is “discovered,” or whether it is invented at the moment one creates/proves a theorem. What does that have to do with the level of creativity involved?
I am a Platonist, and as such, in my view it exists. Moreover, as I wrote above, in my opinion it is analytic and not synthetic, and therefore basically says nothing about the world. But it does not deal with the world; rather, with some Platonic world. Therefore, as I understand it, this is discovery and not creation. It is an observation of that Platonic world, and the laws of mathematics are its “laws of nature.” True, there is a great deal of creativity in this process, but as I explained, that is not relevant to the issue.

There really is no difference between invention and discovery.