Primer advice for wall unit
Use enamel. Latex is too soft for book shelves. Using the right primer is also important and this depends on the material you are trying to paint. You don't want the paint to be removed by placing things on the shelves, so the bond to the base material needs to be strong. I don't think the primer you pictured is an appropriate primer for the job. The primer needs to be as hard as the topcoat or you get cracking in the topcoat. It's a little funny that the Rustoleum website has a video of someone painting a book shelf using that primer. I can't tell you the number of times that I've seen things stuck to shelves because the wrong paint was used. That primer even states that it's flexible... AKA too soft for book shelves.