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?
が～Vてある is used to describe a state that comes from completing V. を~Vてある is used to indicate that V has been completed in preparation for something. In your case, it's that the information has been provided for your edification.
The table from here helps show the two usages of Vてある (though I think it's wrong that ~Vている can only be for intransitive verbs):
And here are some examples, pulled from here and the previous link:
たくさん漢字の練習をしてあるから、今日のテストは問題ないと思う。
A：小林さんに来週の予定を話してありますか。B：いいえ、まだです。
