Learn Polish: Pisanie książek jest trudne. - Writing books is difficult.

You form it by dropping the -ć from the infinitive pisać (to write) and adding the suffix -nie. All verbal nouns formed this way (with -nie or -cie) act as regular nouns and are always neuter in gender.

If "I write books" uses the accusative case (Piszę książki), why does this sentence use the genitive form książek?

When an action becomes a verbal noun like pisanie, the object of that action must switch to the genitive case. You can think of it as the English equivalent of "the writing of books." Since we are talking about plural books, it becomes the genitive plural książek.

Why is the adjective trudne used instead of trudny or trudna?

Adjectives must match the noun they describe in gender, number, and case. Because the subject of the sentence is pisanie, which is a neuter singular noun, the base adjective trudny (difficult) takes the neuter ending -e to become trudne.

How do we get the genitive plural form książek from the feminine noun książka?