Learn Finnish: Luin tiedotteen aamulla sähköpostista. - I read the announcement in my email in the morning.

Because the action is completed. Using the total object tiedotteen (in the genitive form) shows that you finished reading the entire announcement. If you used the partitive tiedotetta, it would mean 'I was reading the announcement' (and might not have finished).

What kind of word is tiedote and how does it decline?

Tiedote is an e-stem noun. For these words, the final -e doubles to create the stem (tiedottee-), which triggers reverse consonant gradation (the weak t becomes a strong tt). Finally, the genitive -n is added to get tiedotteen.

English says 'in my email'. Why does Finnish use the elative case (sähköpostista) here?

In Finnish, verbs related to getting information (like reading, hearing, or finding) usually take a 'from' case because you are extracting the information out of a source. You read out of an email (sähköpostista) just like you read out of a newspaper (lehdestä).

What is the exact nuance of the word tiedote?