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:
「これだけは」にはその項目を教えるにあたって最低限知っておくべき情報を挙げてあります。
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?