being able to open the Logbook in-game

[Huntress]

I can't memorize what and every item does, and sometimes I need to know what they do while I'm playing.

[Nibbie]

They say what they do when you pick them up, and when you hover over them in the tab menu. Doesn't help much for choosing multishop terminals, but it should work for anything you currently have.

[Ver-Anna]

You can press tab to see the vague description of what the item does (the description you get when you pick the item up). You can add the itemStats mod to that if you want a description that literally gives you the exact stats of what the item does (with numbers and everything).

If you're looking for a description of what the item does before you pick it up, I believe you can get that vague description when standing nearby an item using the betterUI mod, but that might only be for lunar items. But honestly, just pause the game and look at the wiki

[Lunacy]

If you play enough you'll learn what everything does. There's also vanilla-legal mods that can change the item descriptions to show exactly what they do.