In Act 4, Scene 2, Brutus tells his young servant Lucius:
Look, Lucius, here's the book I sought for so.
I put it in the pocket of my gown.
He does not say that he had given it to Lucius, but apparently there had been some previous discussion of it. Lucius replies:
I was sure your lordship did not give it to me.
Brutus very kindly and thoughtfully apologizes:
Bear with me, good boy, I am much forgetful.
The main purpose of this discovery of the book in his gown seems to be to establish that he does have a book and that he intends to read. Shakespeare wanted to have Brutus reading when the Ghost of Caesar appears. This would position both the actors on the stage where the playwright wanted them. Brutus would be seated by a candle, and there would be only a dim lighting, which would make the encounter more effective. Since the Ghost would have to enter from offstage, Shakespeare probably intended Brutus to be positioned near one of the wings.
Before the Ghost appears, Brutus says to himself:
Let me see, let me see, is not the leaf turned down
Where I left reading? Here it is, I think.
This would appear to be an anachronism. The kind of book Brutus is reading would not have existed in ancient Roman times. The books were written by hand on scrolls. This copy work was normally done by skilled slaves. Printing presses did not exist. It would have been impossible to put such a scroll in the pocket of a gown. Shakespeare is apparently thinking of a small book made up of printed pages which would slip into a pocket easily (assuming there were such articles of clothing as dressing gowns with pockets on the sides), because Brutus has saved in place in the book by "dog-earing" one of the pages.
Another anachronism in Julius Caesar which shows the lack of knowledge about ancient history in Elizabethan times can be found in Act 1, Scene 2, where Casca tells Brutus and Cassius how Caesar behaved before the crowd:
Marry, before he fell down, when he perceived the common herd was glad he refused the crown, he plucked me ope his doublet and offered them his throat to cut.
A doublet was a close-fitting jacket with or without sleeves worn by men in Shakespeare's time. Many were worn like vests under coats and must have buttoned down the front. Evidently the playwrights and their audiences had no idea how men dressed in Caesar's time, which would have been some sixteen centuries earlier. Shakespeare's ancient Romans may have been dressed more or less like contemporary Elizabethans.