I don't think it helps immersion if there is a 'NEW' or 'UNREAD' glowy tag next to a Table of Contents in the Journal. And it also doesn't help immersion that the journal just automatically grows, like some idiot savant item-based NPC who loves to collect data.
So instead of those mechanisms, picture a guy sitting around a campfire with a candle, book and pen/charcoal, adding to his journal. You could actually see a map in the journal grow as he filled in the details since the last update. Or see him choose to commit to page all he's learned so far about the Mystical Mushrooms of Midlothia.
You as the player would decide which nuggets to write down, and the act of writing them down is what actually gives you your first view of the details. This allows you to have an effective "Unread" tag for the new or unread entries without having it show up explicitly in the world. It would be implied by whether or not your character had written it down.
Managing the unwritten entries would be via the interface, representing the fact that it is information and choices made by the player. It's happening inside the character's head. At any particular point in time you might have 5, 10, maybe even 20+ things to add to your journal. But until your character takes the 'time' to organize your thoughts and put them to paper the details aren't accessible to the player, just the summary that the player remembers when the information was first found.
Getting a little long here, but I just wanted to add that journal entries describing recent events or discoveries is a great way for game designers to draw attention to connections that a player somehow missed. I had a P&P DM who provided a written summary of the previous session for the start of the next session. It was correct (he didn't add anything) but he could draw our attention to overlooked threads in the tapestry just by how he phrased things.