Not a teacher, but a parent of a child with SN who struggles in reading and still guesses words by looking at the first letter.
A speech pathologist put me onto the Fitzroy readers:
The are phonetics based and each book introduces a new sound.
I was told
1. to steer clear of the 'x' series, and just to stick with the regular books.
2. when they can read the level 20 book, they are competent enough to branch out to chapter books.
I have found the Fitzroy readers are available at my local library.
Failing that, I've always found the Aussie Nibbles books great first chapter books. Some are easier than others, so it may pay to scan through them first. They are readily available at libraries.