Adjective for withstanding the passage of time

Is there an adjective meaning that an idea or object has longevity, or will still be relevant in the future?

(I believe I've heard such a word, but I can't think of it at the moment.)

Usage example:

While this textbook is interesting, it does not seem particularly [?]; five years from now, it will be useless.

There are a number of possible synonyms:

eternal, lasting, permanent, enduring, abiding, immortal, everlasting, ceaseless, immutable, indestructible, undying, ageless, imperishable, changeless.

The word required in your example, however, might be definitive, which carries the sense of being authoritative.

Two additional ideas would be:

• timeless ("not affected by time; ageless")
• future-proof ("designed not to be obsolete in the future")

I like enduring; 'twas the first word that entered my mind.

Barrie listed many good candidates, but I'd like to add one more: persistent.

persistent |pərˈsistənt| (adj)
1. continuing firmly or obstinately in a course of action in spite of difficulty or opposition
2. continuing to exist or endure over a prolonged period

The word would be longevous, but this is archaic.