In Hamlet Act 2, Scene 2, Hamlet is reading a book. Is it Cardan's Comforte or St. Augustine's The City of God? What is the clue for the answer?
I am asking this question because I am in favor of Augustine's The City of God as I found this paragraph which is identical to the “To be, or not to be” soliloquy:
I ask whether it is better to suffer one and die or to fear all and live? I am not unaware how much sooner a man would choose to live long under the menace of so many deaths rather than by dying once to fear none of them henceforth, but it is one menace that the aprehension of the flesh weakly seeks to escape and quite another that the carefully analysed calculation of the mind pins down.
Unfortunately, there is no definitive answer to your question, only opinion and conjecture. That's because we only have the text Shakespeare wrote to go on, and the only actual information we have about the book Hamlet carries onstage in Act II, scene ii are these lines:
But look where sadly the poor wretch comes reading.
What do you read, my lord?
Words, words, words.
What is the matter, my lord?
I mean the matter that you read, my lord.
Slanders, sir. For the satirical rogue says here that old men have grey beards, that their faces are purging thick amber and plum-tree gum, and that they have a plentiful lack of wit, together with most weak hams. . .
And this is all we know about the book that Hamlet is reading. Period. As the editors of the Arden Edition of Hamlet point out in their note to the Act II, scene ii stage direction ("Enter HAMLET, reading on a book"), "[a]ttempts to identify the book [that Hamlet carries] are pointless."
Certainly Shakespeare used all kinds of other written sources as material for his plays. We can identify, for example, the source material for many of the plots. So, you are probably on a good track for uncovering from where the idea for text in the "To be or not to be" soliloquy might have been taken. But that, unfortunately, does nothing to prove anything about the specific book held by the actor playing Hamlet as he enters Act II, scene ii.
Decisions regarding props in any play (including Shakespeare's) are not decided by the text, but rather by every individual actor/director staging the performance. And the prop choices for Hamlet's book, made for hundreds of stagings of this play over the centuries, have, I am sure, varied wildly. So, you won't find out what book Hamlet reads until you see the play performed live!