Learn Finnish: Tämä kirja on helppo.

To say 'This book is not easy', you replace the verb on (is) with ei ole (is not). The sentence becomes Tämä kirja ei ole helppo. The word ei is the negative verb for 'it' (the book), and ole is the special form of the verb 'to be' that must be used after a negative word.

Why is helppo in its basic dictionary form here instead of the partitive case?

Because a book (kirja) is a single, countable object. In Finnish, when you use the verb 'to be' to describe one whole countable thing, the adjective stays in the basic form (nominative). You usually only use the partitive case for uncountable things, like mass nouns (for example, Vesi on kylmää - The water is cold).

If I wanted to talk about multiple books, would the word tämä (this) stay the same?

No, it would have to change! In Finnish, pointing words (demonstratives) have to match the noun they are attached to. If you wanted to say 'these books' (plural), tämä would change to nämä to match the plural word kirjat.