Lots of Dick King Smith books are quite short, but are well written.
Astrosaurs are short and have friendly covers.
Captain Underpants books are good - they have little comic strip bits in which help to break them up.
I've just read Fortunately The Milk by Neil Gaiman - very funny, big print (always comforting) and excellent illustrations by Chris Riddell.
Opal Moonbaby books are funny and not too long.
Maisie Hitchens detective stories are also not too threatening and well written.
Maybe the shorter Roald Dahl books, like Esio Trot and George's Marvellous Medicine.
Mr Gum books are slightly longer but also very engaging.
Also, I hesitate to recommend, but the Beast Quest series were the books that really helped my DS to transition from competent reader to bookworm. They're horribly formulaic but they really built his confidence.
It's a shame your library isn't good - I would be significantly poorer without ours! Although the Book People are great for sets of books like Beast Quest.