I wish I (and my wife) could work without books.

Unfortunately we do 300+ songs that run the gamut from those tricky Jazz songs (I find the chord structure in some of those songs to be intimidating) to lyric intensive Rap/RnB
(anyone tried remembering the ENTIRE rap to Bust-a-move????)

We pretty much know our core songs too, but there are requests for stuff (or "different" gig situations we find ourselves in) that require us to do songs we haven't done for AGES.

Ya know, there is even a school of thought I've encountered where some performers think it's only PROFESSIONAL to have your lyrics etc handy because to forget the words of a beloved song makes your audience pretty unhappy, and does make the performer look silly...

It's worth noting that we use almost entirely SMFs - so the arrangements are set in stone, not arranger styles, where the PERFORMER can dictate the way the song goes (if they, say, forget a verse) so it really is a "discipline" to play with a sequence in that regard.

In short, we have our books within "eyeshot" and keep them unobtrusive to the audience.

We get the words right.
We get the arrangements right.
We do, honestly, a STAGGERING range of material.

All thanks to the books.