Should I use "textbook" or "text book"?

Textbook is always correct. Text book is incorrectly used by people instead of the compound word 'textbook'. The plural form is 'textbooks'.

Examples of textbook in a sentence

And they're also in a pretty standard textbook order."
- Cerulean Sins by Laurell K. Hamilton

He looked up from where he was torturing a page of his textbook.
- A deeper sleep by Dana Stabenow

From what I remember, the Manson case was textbook disorganized crime.
- Exit Strategy by Kelley Armstrong

Textbook asshole.He smiles.--Ever
- My Dead Body by Charlie Huston