From comment:
I like to write the stuffs down on the book when I annotate a book.
First of all stuff is a mass noun and not used in the plural. If someone said stuffs they were using the word in a rare and unusual manner, perhaps humorously. (The word foodstuff is used as a count noun.)
Here, in both cases the is used to signal to the reader or hearer that he or she should be able to identify which stuff and which book the speaker is talking about. The speaker assumes the hearer can identify which stuff and which book he means. As a matter of fact, I personally cannot do this, and I have no idea which stuff and which book are being talked about. There would have to be some other way–existing outside of the text of this sentence–for me to identify which ones he means.
One of these ways is that the speaker could have mentioned them before. This is where the so-called first mention guideline comes into play (I could never call it a rule because it simply isn't.)
Another way is that he is holding the book in his hand and obviously referring to it as the book. Again, this is not so much an emphasis but a subtle signal: there's no other book in sight, so he must be talking about the one he's holding. In this case he could also say this book.
Likewise, perhaps it is obvious from context which stuff he is talking about. For instance, if this sentence was said during a video and it is clear which book and which stuff the speaker is talking about, then the speaker can use the definite article with them.