I agree, that book is a great one to have , another on that i love is:
the green tree python and emerald tree boa by Andrea kirshner and Ron Kivit.
it's one of the only good books out there that guides you on ETBs also.
as for cage sizes, crotalusfan is right, width is better, the ideal size cage for a full adult would be 24x24x36L. Most people will go with a 24 cube witch is also perfect, if you have the space and $$ go bigger for adults , they will enjoy the room and it's good excersize for them.
I have a huge 8' female ETB in a 24x24x36 cage and she loves it, but in her case and at that size, if i had room for a 36x36x48L she would love that.. here's a bad picture of her in that cage.
care wise, both the GTP and ETB are are almost the same.. the main diference is ETBs are more delicate to not so perfect conditions.. humidity shouldn't go under 60% and temps aren't as high..
I wanted to mention that for heat panels, i have both pro-products and RBI , from my research and use of them, i think they both do a great job.
the pro products seems to be of better quality versus the production ones from RBI, but in the end they do the same job..
I'm sure you will do great with your GTP, just be mentally prepared for what comes next!!!
keeping just one is next to impossible LOL