This is Endymion by Dan Simmons, the third in a series starting with Hyperion.
“Back in the late 80s or early 90s I read a series”
The four books in the series are Hyperion (1989), The Fall of Hyperion (1990), Endymion (1996) and The Rise of Endymion (1998).
“about a galactic empire that used ground based portals to travel between planets. There was some ...
The Hobbyist by Fredric Brown
Brown's flash fiction short story "The Hobbyist" (1961) is about a man
named Sangstrom, who is in a desperate search for an undetectable
poison but winds up getting more than he bargained for.
Following the Ngrams hint, I eventually located an illustrated 1903 edition of Tales from Maria Edgeworth. Page 91, near the beginning of the story "Simple Susan", contains an illustration by Hugh Thomson matching the description of a "farm half hidden by trees" in The New Treasure Seekers:
This story had been in publication since 1796, ...
Sunny the Greedy Goat Learns the Value of Self-Control by Ethel Barrett looks like possibility:
A greedy little goat learns to control himself after suffering the ill effects of an eating binge.
The above summary is quoted from the book's entry at WorldCat.
There are few images available Chinese website 258sd.com.
Is it The Island of Adventure (1944)?
This is the first book in the series. Some children explore an island near where they are staying, and discover a counterfeiting operation in some copper mines. The tunnels connect to the well at their house.
Excerpts from Anita Bensoussane's review via The Enid Blyton Society:
[T]he children visit the Isle of Gloom, ...
Here’s a notable early example of the phrase being used in this sense:
A certain robot, planning to go on a long and dangerous voyage, heard of a most useful device which its inventor called an electric friend. He would feel better, he thought, if he had a companion, even a companion that was only a machine, so he went to the inventor and asked to be shown ...
I have been unable to find a book entitled Sensible Susan in the catalogues of the British Library, WorldCat or the Library of Congress. Due to the alliteration in the name, it is also possible that "Sensible Susan" refers to a stereotype that figures in a specific book and that the characters in Nesbit's The New Treasure Seekers are referring to ...
The first one sounds like it might be one of the Murderous Maths series of illustrated maths books for children by Kjartan Poskitt. I found a sample illustration related to the Fibonacci sequence and golden ratio, and also a picture of the book covers (not all of them as there were also two books named simply Murderous Maths and More Murderous Maths before ...
Sources for the Newgate Calendar claim
In a comment, Spagirl tracked down the following review of a 1924 performance of the play:
We have been exhilarated by slightly cooler weather in the desert, and the other evening Gordon, our djinn, took the Princess and ourselves on his broad wings and wafted us in the wink of an eye to Manhattan, depositing us at the ...