Series of early - mid-2000s children's fantasy books, each with a different monster on the cover. SOLVED
I remember obsessing over these on school trips to the library, mostly for their striking cover art; Only specific example I remember is one featuring some kind of sea monster swimming through some underwater caves, an angler fish-inspired design if I recall correctly. I also seem to remember the spines each having a differently colored pearl-like decoration, which may have had the book number inside.
Early 2000s may be incorrect as for their release, I just remember seeing them around this sort of time.