there are quite a lot of different ranges
you have Oxford Reading Tree ones which include the classic stories/Biff Chip and Kipper, and they also have Songbirds Phonics (which I personally think are excellent phonics books) and Read Write Inc are supposed to be good but my elder daughter didn't take to them. google Oxford Owl and you can read some of these online so you can see what the style of them is at different stages and decide if they might suit you.
there are Collins Big Cat ones which we have a few of at different levels but I am not sure I would recommend them as a series for learning to read, nice for odd 'other books' to use different styles though
Usborne have some different ones too, as mentioned the 'easy words to read' ones like Shark in the Park are good although there are so many sounds in a small number of books they can be quite heavy going at the beginning for kids learning to sound out. They also have another series which I can't remember the actual name of which follow through different levels too.
Not helpful if they are learning purely phonetically but my daughters love the Peter and Jane books and very quickly learned a lot of the high frequency words, they CAN be sounded out if they know the phonics for them but if they learn to read them fluently then it can make life a bit easier for them. HOWEVER it can confuse some kids so it depends on how your child takes to reading.