I read this when I was in middle school in 1983, so it has to be from that year or earlier.
The story is, four children: Nels, age 12, Stevie, 9 (his turning ten is a big plot device), Rory, 8, and Jenny, 6, are spending the summer at an inn owned by their aunt and uncle while their parents’ divorce is being finalized. The two younger children are happy as clams playing with the housekeeper’s daughter, but the older two are very disaffected. Nels is getting into that moody teenager thing, and Stevie is all needy.
Nels finds a closet in one of the rooms that opens onto a tower. A guy his own age, who is perfectly on his sci-fi geek wavelength, lives in there with his wonderful, perfect parents. The two of them hangs out in the tower every day. The aunt and uncle don’t seem to know anyone lives in the tower, and Nels doesn’t tell anyone he has this friend.
Meanwhile, Stevie is completely out of the loop, so he finally ends up going to the park nearby where some older teenagers hang around; they kind of humor him, which is better than nothing. The aunt is impatient with him, the uncle is a space case, Nels brushes him off and ultimately blows up at him. Stevie just gets no love! He’s looking forward keenly to his tenth birthday, and on the day, he gets caught sneaking a box of ice cream bars out of the inn’s store so he can host his own birthday party. Of course, this is the first time anyone remembers that he was having a birthday, and Nels is overwhelmed with guilt; how could he have forgotten his brother’s birthday, when he even remembers the birth day?
Before the impromptu birthday party, Nels goes to confront Alan and ask him once and for all: who is he, and how can his family live in this tower without anyone knowing about them? Alan won’t give him an answer, and when he asks Alan’s parents for an explanation, they fade away like apparitions. Then he can’t get out of the entrance room. Stevie finally hears him pounding on the door and lets him out. Turns out…there IS no tower. He must have hallucinated the entire thing.
Now, SOMEBODY has to remember this! I’m stuck on the title and the author, except that the author’s name was somewhere in the middle of the alphabet, maybe M-something or Mc-something, because I can picture the book being in that part of the stacks.