A journal page is a page of the dominion journal. There are 20 pages total. You can obtain all the pages from one mode.
It seems that the chances of getting a page increase the higher one's dominion factor gets. An effective way to do this is to do Climber Mode and fight to as high of a floor as possible since your dominion factor accumulates very fast in this mode. Reaching floor 20 will usually yield over 200,000 dominion factor, which usually results in one or two journal pages upon opening the rewards box. However, if you come across a boss you don't think you will be able to defeat, it is better to end climber mode and collect the reward than to die fighting the boss as your dominion factor will be greatly reduced upon death.
If one completes the Journal (by adding the pages to the Dominion journal) and stores it in a player-owned house bookcase, you will start to receive pages again. It is up to the player whether they will try to complete a second journal or just destroy the pages.
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