写道could means "mention" 例: 在这本书中写道:鲁迅的人生是多彩的 eg:this book has mentioned that LuXun has a colorful life

"写道:......." =" it's written like this:......."

if the sentence is "书中写道"，then 写道 here means "said" i think. for example,书中写道=the book said

Hello Michae, 写道 = to write down

I give you the simplest answer: it is written by sb/ in the sth (book, newspaper etc) that.... and by the way, it's a very formal expression, if you use "写道" when speaking to others, they would think you're a pedant, hehe!