I read these three books when i was much younger. They were British mysteries and they were illustrated. One was about a marsh. The townspeople think there is a killer on the loose but the culprit turns out to be the marsh gases. A botanist is confused by delirious victims as the killer.

The next is about a mansion and two siblings a boy and a girl being send off to visit their distant relatives. There they encounter mysterious happenings including a clock going off at strange hours I believe. The killer turns out to be the butler who was actualy the heir to the fortune but gambled it all away.

The last is about a mysterious painting with a woman in a red cape. she is smiling but her posture often changes. they say she leads ships to their death. They can tell by the way her picture is. There is a man who specializes in creating antiques. he duplicates them.

All of these books have intricate illustrations and are full of "red herrings". They have clues at the end of the book and the detailed answer on the the guilty party is and why. Anyone who knows these I would be so grateful. I have all this from memory but no titles or author. i'm pretty sure it was a series but with different characters each tiem.