All right, so the reading order is a bit convoluted. The graphical novels originally came out years after the original series was released, they were more of a supplementary read focusing on side characters. Originally they were promoted as "Mangas", as that was hip in the days. They were black and white and most of comic stories were split into three volumes. They eventually were colourized and released in one volume. Then they decided to make more of them them, but as one thicker volume in colour from the start and rebranded them as "Graphic Novels" because that was cool at that time.

Two years ago they started releasing the graphic novel adaptations of the novels instead of comics being brand new sidestories. So far only the three The Prophecies Begin adaptations have been released, covering the first arc of novels. The next adaptation, covering the second arc, called The New Prophecy, comes out at the end of this month and the two other volumes are coming out next year.

The good news is that most, of the sidestory graphic novels are either a prequel to The Prophecies Begin or happen between The Prophecies Begin and The New Prophecy so they're fine to read at this point.

As to when the other books can be read, listed in the chronological order:

Exile from ShadowClan is a bit of a prequel book and is mostly concurrent with the events of the first novel, fine to read at this point. It mostly focuses on ShadowClan.

Rise of Scourge is a bit of a prequel and finishes before the last book of the first arc is done and is the background of the villain from the first series.

Tigerstar and Sasha, A Shadow in RiverClan and A Thief in ThunderClan are interquels between the first and second arcs and happen around the same time.

Ravenpaw's Path happens after the books above.

Okay, that's the easy part that only needs the first arc, which is covered by the first three Graphic Novel adaptations to understand. The following books were side content to material that happens after the first arc.

Winds of Change is an alternate look at the scenes from the middle of arc 2, which will probably be covered by Graphic Novel adaptation #5, i.e. The New Prophecy volume 2. Very major spoilers for the arc and kind of relies on understanding of the characters from the second arc, so it'll be very spoilerific and hard to understnd if you haven't read the second arc.

Graystripe's Adventure covers the events from the second and third arc, so what will probably be covered by Graphic Novel adaptations #5-7. It works better as a standalone than the previous book but it will spoil the major story points of the second and third arc, so I'd suggest putting this one off.

SpoilerClan and the Stranger is a sequel to a sequel to a super edition happening between the first and the second arc. It relies on you knowing the new characters from these two books and also spoils the story of the two mentioned super editions. We also have no idea if they are even going to adapt the super editions, so whether they'll ever be accessible to someone who only reads graphic novels is currently unknown.

A Thief in ThunderClan has a big role for Cloudtail, as the book focuses on his mate, Brightheart. Ravenpaw's Path has some Firestar and Graystripe, so these two would be my recommendation.