Seen in the introduction to a book entitled 日本語文法ハンドブック, so it would greatly surprise me if it's a typo. Each section in the book is broken into parts and one is entitled "これだけは", being described here:
The end of the sentence is a Vてある construction, but uses を instead of が or は. My understanding is that this is not correct. Is this a typo, or a usage I'm not familiar with?