Learn Russian: Это книга про город.

про is another very common way to say 'about,' especially in everyday Russian. The main difference is the grammar: while о takes the prepositional case you are learning now, про requires the accusative case.

If про takes the accusative case, why hasn't the word город changed?

город (city) is a masculine inanimate noun (a non-living thing). In Russian, masculine inanimate nouns do not change their endings in the accusative case; they look exactly the same as they do in the dictionary (nominative) form.

How would this sentence look if I used о instead of про?

It would be Это книга о городе. Because о forces the noun into the prepositional case, you have to add the -е ending to город. Both versions of the sentence mean the exact same thing!

Why do we use Это instead of the feminine Эта to match the feminine word книга?

In this sentence structure, Это acts as the phrase 'This is'. When introducing what something is, you always use the fixed word Это, no matter the noun's gender. If you used Эта, it would attach directly to the noun to mean 'This book' (for example, Эта книга про город translates to 'This book is about the city').