Airflow - must have airflow to avoid mould.
If your shelving is 250mm deep Mike, there will be no gap behind the majority of books.
300mm deep shelves can cope with 99% of a typical book collection and provide that essential air gap for most.
Some forced ventilation might help your situation - the shelving and books have been effectively sealed up by your plywood doors.
If there is room to do so, what about installing one of those really quiet bathroom extractor fans at one end, with the outlet at the far end?
You can put it on a programmable time switch so that it runs say twice a day for 15 -30 mins to change the air.
I've done this with great success in storage containers.
It all depends whether air can circulate
behind the books I guess - as a last resort you could drill a series of large diameter holes at the rear in the metal shelves, say 50mm dia. or more, to provide airflow.
I had originally specced up plain shelving as being the more aesthetically pleasing of the two options (plain or slotted) but this had been discontinued and slotted shelving made standard, although the manufacturer had just enough stock left for what I needed so I went for plain.
But then I got thinking about air flow and realised that the slots might assist with airflow so plain might not be that sensible for the very reason you have discovered.
I also found I could get dividers for use with the slotted shelves and realised that once the shelves are full of books the slots cannot be seen anyway - dividers would solve the magazine storage issue. Order changed!
The dividers have two tabs at the bottom that locate as per arrow in the pic below: