If the modpack ships without default keybinds for those actions, then by default there won’t be a keybind to flip to page 3 (casting the spell could be the “use” function for the speedbook). Flipping between pages would be part of the action menu by default, right?
So by default, the player picks up the spellbook and has to press ‘f’ to bring up an action menu, then move the mouse cursor over the page they want to turn to (or the spell they want to cast), then left click to select that action, then right click to cast. All of this will happen in the heat of battle.
Yes, the user is free to bypass the action menu with keybinds. It will probably only take a minute to set that up. But what you are saying is that modpack and mod authors are powerless to make the default process any less painful.
There’s a problem, however, if you have multiple mods adding their own multitools. Let’s pretend this roguelike pack includes both the speedbook and the swiss army knife. Players will use both tools quite often. The modpack author would like it to be so that the keyboard’s number row flips to pages 1-12 of the speedbook or switches the mode 1-10 of the army knife, depending on which is being held. But the modpack is not allowed to ship default keybinds for action menu items, and neither are individual mods. That means the player has to go to the trouble of assigning 12+10 = 22 keybindings.
Do you see how this might be a problem if a modpack includes lots of multitools? You might end up with hundreds of action menu items that the modpack author knows will fit on just twelve keys without conflicts. But according to your rules, the player has to personally set every keybinding.