Lots of great books, of course, but I am particularly fond of Jesus, Present Before Me: Meditations for Eucharistic Adoration by Fr. Peter John Cameron, O.P.
Then again, for Adoration, I like to bring my breviary, mostly, as it is a chance to pray an hour (usually Vespers) before the Blessed Sacrament. I also like old-fashioned prayerbooks like this one. They’re kind of comforting and familiar, even if the language is a little stilted.
And I have heard this is good, but haven’t used it.
Hope that helps!