The answer above goes in depth so I will try go off that.
Firstly, lamella is the word used to describe plate-like structures. A thylakoid therefore, being a flattened vesicle, would fit this description, so a thylakoid is a type of lamella.
As mentioned earlier, a lamellar system consists of uniform thylakoids, the thylakoids being the individual lamella.
I have the same textbook as you, OCR A2 Biology, and I must admit I had the same confusion. What this textbook fails to mention is the fact that thylakoids are just one type of lamellae.
The textbook also states that the inner membrane of the chloroplast is "folded into lamellae (thin plates)". There are two types of lamellae in the chloroplast, thylakoids and intergranal lamellae, both are stacked, however only thylakoids appear as "piles of pennies" as it states. Intergranal lamellae look nothing like pennies, which is why the textbook is strongly misleading, they are big flattened bits of membrane that connect together different grana (stacks of thylakoids).
Note: my answer comes in 3 years late (I'm currently doing my A2), and the textbook in question will see its last use this year as the current A2 level syllabus is updated, I hope my answer still helps to clear up any confusion should OCR mess up again on this one