Due to my mother's computer restrictions, I feel like buying a z80 ASM book from Amazon would be better.
Are these books good: http://www.amazon.com/dp/0471081248/ref=pe_385040_128020140_TE_3p_dp_1?
http://www.amazon.com/Programming-Z80-Rodnay-Zaks/dp/0895880695/ref=cm_sw_em_r_tt?
Alternatively, if your mom or school allows you to use a printer, you could print a copy of ASM in 28 days and Hot Dog's complementary tutorials, but the latter is quite long and uses a different way of naming stuff so it might not be ideal.
I never checked those books out, but ideally you should also use a tutorial specific to calculators (which doesn't exist in book form), because some things are different on the calculator compared to other Z80 platforms, especially hardware-wise.
Those books actually helped me with z80 Assembly a lot
You should try to use the knowledge acquired from this book and practice by writing your own code or experimenting with the code in the book, but keep in mind that if the book is not calc-specific, some things might be different, so always keep copies of WikiTI, Asm in 28 Days and Hot Dog's ASM Guide for the Absolute Beginner nearby as reference.