Speaking to Sky before Borussia Dortmund's 2-1 pre-season defeat to Fortuna Dusseldorf, Book outlined the club's remaining transfer priorities without naming specific targets.
"We know what we want. Those are two attacking positions. We're very happy that we've been able to hold the spine of this team together," Book said.
"We know that we still want to do something, but at the end of the day we want to do the right things. That's not just about pure speed, first and foremost."
Book also acknowledged that Dortmund, last season's Bundesliga runners-up, would have liked to have already added more firepower to their attack at this stage of the window.
"Of course we would have liked to have already signed an attacking player, but ultimately you have to pay attention to the laws of the market and move with the times. Timing is absolutely crucial there," he said.
Pressed on whether Said El Mala, the 19-year-old Koln winger, was part of those plans and potentially the second attacking signing Dortmund are seeking, Book declined to be drawn on the specifics.
"Basically, I think everyone is aware that we don't comment on players who are not players of Borussia Dortmund," he said.
Dortmund are understood to have submitted a formal offer for El Mala to Koln, a proposal that reportedly caused irritation at the club given the size of the bid relative to Koln's valuation of the player.
Communication between the two clubs has since gone quiet, with relations needing repair before any renewed negotiations can take place. Karetsas, meanwhile, is expected to complete his move to Signal Iduna Park in the near future, becoming Dortmund's latest young signing as the club looks to add further depth and quality across its forward line before the transfer window closes at the start of September.
The Greek international has impressed for Genk over the past two seasons, and his arrival would represent a significant statement of intent from Dortmund as they look to build on last season's runner-up finish and mount a serious title challenge.