Robin tome system is odd...
They will have a limited supply as demonstrated with Robin throwing the book away and becoming empty handed
We can at the least cycle though the spells as in the last part of the video she taunts with the green book, the taunt completes with it still green, she uses arc fire, the book is red, she rolls with the red, then she uses arc thunder, the book turns yellow.
There's four variations which is shown with thunder, elthunder, arc thunder and the strongest lighting attack which I forgot the name off. We also see near the end with all four FE characters Robin getting ready to use a 'big' fire attack which makes me think we will have four variation of each tome book.
Last we are shown a very clear charge up animation
Conclusion: With everything I've pointed out put together it makes me think we will have either free control or be on a set cycle of tomes we use and that charging up will be the only way to 'upgrade' your tomes to the next level but by doing so will drain your tome supply faster.