"coiner" - one who counterfeits coins. Not something we hear of much today, but in earlier times was a reasonably familiar sort of criminal.
"box-room" - a room in a house (usually a small room) which is just used for storing things, usually things that are not wanted very often. The collocation "box-room attic" is unusual, because these two words mean nearly the same thing. (Not quite, because a box-room could be anywhere in a house, not just under the roof).
"cistern": yes, it is still quite common for British houses to have a water-tank in the roof space - sometimes two (hot and cold). This was partly to provide a head of pressure for taps and showers, and partly to guard against interruptions in supply (and in the case of hot tanks, to provide a reservoir of heated water for when you needed it quickly, as in a bath).
There is not a literal tunnel, it is saying that between the tank and the sloping roof there is a long and narrow dark place (dark because it is behind the cistern).
"of course" is a parenthetical remark meaning "as you already know", or "as is obvious". It is much more common in speech than in writing, but of course this writing is meant to suggest somebody is speaking, and telling a story.
"For" says that the sentence is a reason or explanation of what precedes. So the meaning of "For of course he was thinking .. " is something like "(He was excited) because, as you probably realise, he was thinking ... "
'swish' is just onomatopoeia for the ball not touching the rim, only the net as it goes through, making a swishing sound. It's not a noun or verb. It's the word for the sound it makes.
The implication of the word is that Sheldon did something really well, something without any flaws at all. It is not a common thing to say, but is immediately understandable.
Another similar thing to say with the same meaning is descriptive "nothing but net" (much more common).
Best Answer
"This [object] is anything but" is another way of saying "This is nothing like what I had initially thought about .. [that object]"
So the author thought the book given by his friend would not be to his liking, as the donor was "heavily into the technical aspects of architecture" and he was pleasantly surprised when that was not the case. The book was not as bad as he thought.