>>> aimed at the American style of learning

Ouch! Boas is maybe not as inspiring as Feynman. But when you see a copy on someone's bookshelf. It tends to be just as dog-eared and spine-cracked as Surely You're Joking

Another resource I just thought of. While not a textbook per se. Math competition problems from previous years can be very stimulating ;)

https://www.maths.cam.ac.uk/undergrad/pastpapers/past-ia-ib-...

https://kskedlaya.org/putnam-archive/