Questions & Answers about Eikö tämä kirja ole kallis?
How exactly does the word Eikö work in this sentence?
Eikö is made of the third-person negative verb ei (is not) and the question ending -kö. Attaching the ending directly to the negative verb and putting it at the start of the sentence creates a negative question, meaning "Isn't...?".
Why is the verb ole used instead of on?
Because this is a negative sentence. Whenever you use the negative verb (like the ei hiding inside Eikö), the main verb olla (to be) loses its personal ending and must be in its negative form, which is ole.
Why does the question ending use an ö (-kö) instead of an o (-ko)?
This is because of vowel harmony. The word it attaches to is ei, which contains only front vowels (e, i). Because of that, it requires the front-vowel version of the question ending, which is -kö.
Could I start the sentence with Tämä kirja to match the English word order?
Not if you want to ask a yes/no question. In Finnish, the word carrying the -ko/-kö question marker must come first. If you say Tämä kirja ei ole kallis, you are making a statement: "This book is not expensive."