The four colored tome/books from AtS have me a bit confused. When I discard to pick up an event from the discard, I must "play as if it were in [my] hand." Does this mean I have to pay the cost listed on the event?
Yes, you must pay the cost.
Also, would action windows allow for me to use a Feint, discard, then use the Tactics Tome to pick it back up and feint a second enemy in the same combat phase?
Yes. The only Event cards that are not playable are Response cards. This is because the books are Actions, which can't be triggered as a Response.