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I can't retain my attention when I read novels.
I can't sustain my attention when I read novels.
I looked up both words' meanings and, in this case, both should mean the same thing, that's what I think. But, do they mean the same thing here, and are interchangeable?
I don't find "retain" idiomatic in this context. The use of the words is consistent with imagining "attention" as something you hold up, like a flag in your hand.
"Retain" means "keep", or "keep hold of", but has no implication of keeping the object in use, or holding it up, merely continuing to possess it: it might be in a store, for example. I can imagine it being used of the thing the person is focussing their attention on: Novels can't retain my attention, but not of the person attending.
"Sustain" means "keep something going" or "hold something up", and fits well with the metaphor.
The book keeps, retains, or sustains the reader's attention or interest.
If you're referring to an inability to stay focused when reading any book, and it's not the fault of the particular book, you could use the verb maintain:
She could not maintain concentration when reading.
