You are looking at a high-end e-reader because you want something that lasts for years, reads easily in bright sunlight, and holds thousands of books without a single creak in the plastic. These devices do not need yearly upgrades, so if you buy the right one now, you are buying once instead of replacing a cheap tablet in twelve months.
Timing matters here since Mother's Day and Father's Day fall right around the time manufacturers tend to refresh their lineups. Keep an eye out for last year's model going on sale the second a new version drops, because the specs are usually almost identical while the price takes a nice dip.
Remember that the intro window is a strict deadline, not free money, and any leftover balance when those 15 months end will suddenly start racking up standard interest charges.