"I read the book."
Translation:Ik lees het boek.
No, het and de can't be used interchangeably.
But, unfortunately, there are no steady rules for most words, except for:
• all plurals take de as a definite article.
• all diminutive singular nouns take het as a definite article.
Other than that, there are some guidelines you can follow, which you can read here:
And there's an app you can use to check which article to use when in doubt (it also has the guidelines): https://play.google.com/store/apps/details?id=com.piterwilson.deofhet
In English, this could be read as either present tense or past tense. If you're asking "Have you read the book?" to which the answer in this case would be "I read the book". If you would like the Dutch answer to be active tense i.e. 'Ik ben het boek aan het lezen' the more appropriate English sentence would be 'I am reading the book'. Just a thought!