I borrowed this book from the library, probably during the 1970s, but it may have been published earlier. My vague memories include: two children, probably a boy and a girl, had magical adventures. There was a friendly witch or female magician and a good but scary male magician who was always invisible. This may have been by choice, and there was a backstory of a romantic relationship with the witch. One of the children was transported by the invisible magician and found it rather frightening but eventually trusted the magician. This was not the main plot!
The book was almost certainly British and would have been a chapter book (not a term used at that time).
Thank you!
Possibly Timothy and two witches by Mararet Storey – or one of the other books from the series. About Timothy and Ellen. The good witch is called Melinda Farbright. There are 7 books in the series. Timothy and two witches; The stone wizard/The stone sorcerer (UK & US titles); A quarrel of witches; The sleeping witch; A war of wizards and The double wizard.
I think you might be right! I’d love to read them again to find out, but they are very rare and expensive. If this is the right series, I think the one I want to read is one of the later ones. Thanks!