In that case, the correct English translations would be- 1) We write IN the books 2) We write ABOUT books In English, if you say we write ON the books, it would mean either that we are writing on the COVERS of the books OR that we take a piece of paper and place the paper on the book and then write ON the paper.
Which preposition to use is never clean-cut between languages. Neither in meaning nor in usage. You just need to memorize which preposition is used where.
There are, however, clear rules regarding when to use su vs sul vs sullo and when to use in (not "ne") vs nel vs nello. They are directly related to the use of the definite article.
christy: Prepositions are tough in most languages since they have both literal and figurative meanings. Basically though 'su' means 'on' combining with the definite article to become e.g. 'sul tavolo'/'on the table', 'sullo zaino'/'on the backpack'. "In' basically means "in" and combines to form e.g. 'nel' / 'in the' so e.g. 'nel museo' /' in the museum'. (Ne is not a prepositonal contraction). "nella" / "nella scuola"/ 'in the school'.Etc.
No, they are not interchangeable.
The root of all of them is which form of "the" you need to use.
Rules for the definite article:
Then they fuse together with the various prepositions: