I read these books in the late 1980s, no earlier than 1987 and no later than 1988.
The setting of the books is the modern world, but with a ghostly or hidden world intermingled with it - in a similar vein to the wizarding world in Harry Potter and London Below in Neil Gaimans Neverwhere, the hidden world is just around the corner, or just out of sight but always there.
I'm pretty sure the setting was the United Kingdom.
The inhabitants of the other world may or may not have been full ghosts, or they may simply have been magical beings hiding from modern humans. Modern humans may be befriended by inhabitants of the other world, and introduced to various fantastical things, but these modern humans may not be accepted by everyone in the other world.
In one story, the protagonist, a normal human child or teenager, has to travel a long distance and his other world friend arranges for him to take the bus in the other world - in order to be accepted by other world inhabitants the traveller has to take some sort of potion which makes him look like an other world inhabitant (but doesnt change their physical features). Of course, he falls asleep on the bus and wakes up later having forgotten to take another dose to find the potion had worn off and everyone on the bus was now hostile to him.
In another story, protagonist, again a normal human child or teenager, is told they are in danger by their other world friend, and they have to kit out their bedroom with locks on the doors and an arrangement of string from the light switch by the door to their bed so they can turn the light on from the bed. There may or may not have been a sub-story in this one about an angry man losing their wallet, only for the protagonist to find it and then be accused of stealing it.
Other plot points in this book series included magical animals that could talk (cats etc - familiars probably).
These books would have been an illustrated type, probably with half the page as text and the other half as an illustration - suitable for 8 - 10 year olds (that might not be a good estimation on my part).
