I would say that, on the whole, Albertine and What to do with Daylight are much more similar than either of those two are to Flags. Before the album was written and produced, she was on a short break from music and this is alluded to in her song 'Coachella', a song with the same name as the festival which reignited her love for music.
The album is her first to be self-produced and well produced it most certainly is! Apologies for those who are not familiar with her music, I guess I've assumed that a bit here. To help you have a feel for what her music is like, I would say she sounds similar to Norah Jones crossed with KT Tunstall and Nerina Pallot.
Flags is a much less guitar-driven album than its predecessor. While some songs such as 'Jack Kerouac' and 'Coachella' wouldn't feel out of place in Albertine, others such as 'Ice on her Lashes' and 'Crows + Locusts' display Fraser treading different paths to the ones previously walked on. While nothing on the album could be considered groundbreaking, it is certainly a welcome addition to anyone's musical collection. The greatest strength of this, and in fact all her albums, is their lasting effect. While many other albums I've purchased over the years (some from britpop revival, pop punk, emo, dubstep etc.) have now passed their sell by date, this album is most certainly not going to be seen in too many car-boot sales. If financially struggling, I think I'd sell my clothes, furniture and TV before selling this gem.
For a preview of the album, I'd recommend the songs 'Betty' and 'Orphans, Kingdoms' to give you an idea of whether or not to purchase.