This one was known to me years ago, but I don't recall much so I'll do my best in detailing what i remember:
- Most of the cover used in the series had a dark blue color in the background, with some other containing more color/being colorful.
- The "talking" dog usually had weird clothes or apparel on him (pretty sure it was a male dog). I vaguely recall the dog having a monocle/or glasses in one of the book series
- I don't exactly recall any of the story used in the book from the said series, but I do recall it had all the usual/recurrent ingredient/stereotypes from certain TV trope (which I'll detail on more below)
- One enemy in the series was being called "shadow" or coming from the shadows...Not sure of their exact name but it was a recurring theme in one of the story at some point
- The cover of most of the book in the series have the girl and dog being drawn in the middle with varying expressions
- The dog could sometimes see or hear things the girl couldn't (eg: hear voices, see strange figures etc)
- The dog sometimes was seen wearing human-like clothing that was fitting to him
- In one of the book, I vaguely recall the dog wearing a kilt (red/with red pattern on it)
- The girl sometimes had a magnifying glass in one of her hand
- The girl isn't tall at all (mentioning the covers depicting her since I don't know the detail of her exact height) and the dog is mostly on all fours, even though he seems to show human emotions/tendencies...
Details on the "stereotypes":
- There was one book in the series where you could clearly see the dog becoming "evil" or at least, foreshadowing that (based on the cover where the shadow/figure of the dog was drawn, and the summary of the said book described the girl noticing the dog "acting" strange)
- A Talking dog
- A group of individuals solving mysteries
That's all I can remember. I don't recall how many book there is in the series, but I know there more than 4 at least...
I'm unsure if the dog was clearly talking to the girl or "talking" as in, communicating to her in other way than speak, or if the girl just understood him and interpreted what he meant (but it was described so well, it looked like he was talking, so It's hard to say given it was a long time I read this).