For some reason, the report button doesn't let me flag this as "The English sentence is unnatural". It's not quite grammatically incorrect, but certainly sounds archaic, especially when talking about something as mundane as books.
Keeping the same sentence pattern, you could try "My books are few in number" or "My books are scarce".... it's a tricky one.
Wordplay, yes; I was thinking of this kind of syntax - also "my pencils are ten" - as "riddle English", but I am drawing a blank on specific riddle examples. A search did turn up expressions of the "X are few, Y (are) many" variety. "Biblical English" is also a place this finds a home, as in Matthew 37, "The harvest is plentiful but the workers are few." Such phrases take advantage of the "unusual construction", as you say, for their particular expressive or memorable qualities.
I suspect that memorable quality may also stem from an overly literal translation of Hebrew or Greek, if it is not found outside of the Bible. It is satirized beautifully in the killer rabbit of Caerbannog scene of Monty Python and the Holy Grail, when Brother Maynard, reading the instructions for the Holy Hand Grenade of Antioch says, "First shalt thou take out the Holy Pin, then shalt thou count to three, no more, no less. Three shall be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, neither count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then lobbest thou thy Holy Hand Grenade of Antioch towards thy foe, who, being naughty in My sight, shall snuff it." Of course, this is funny precisely because this very peculiar style of English is only seen in the Bible and translations modeled on it.
I'm just a learner, like you, but a number of other languages treat plurals like this. The plural is only used when you need to specify, so general statements will always be in the singular, for instance, and and if there is an indication of quantity ("few" here), it will also always be singular.