Ah yes, the Kindle app doesn't let you copy text straight-away.
There is a nasty little work-around, but it is only advisable for longer text passages:
-take a screenshot, by pressing Power+Home button simultaneously, preferably while in landscape mode
-open an OCR tool, such as 'Image to text - ocr'
-import the screenshot into the OCR tool
-depending on the OCR app, will the text be shown in the app itself, or emailed to yourself, image to text - OCR does the latter.