What does 〜て おきます mean?

[Hyperworm]

A little more on past + tsumori + present

(Stopped and questioned about a book you're holding just as you exit a shop.)
"Didn't you just steal that book?"
その本を買ったつもりです。(=I hope you have some good proof to the contrary, because as far as my perceptions and memory tell me) I believe I bought that book. / I'm pretty sure I bought that book. / (That's funny 'cause) I'm under the impression that I bought that book.

(A teacher wondering why you don't have the book in your bag.)
"You probably spent the money on drink instead of the course textbook, didn't you?" （ひどい言い方ｗ）
その本を買ったつもりです。I bought that book, as best as my memory recalls... (I wonder why it's not in my bag...)

etc.

[ss]

Thanks Nilecat and Hyperworm for quoting a possible scenario to make it perfectly understandable.