The time I spent in the library was a most rewarding one.

profitable : (of a business or activity) yielding profit or financial gain.
paying : give (someone) money that is due for work done, goods received, or a debt incurred.
serviceable : fulfilling its function adequately; usable.
precious : of great value; not to be wasted or treated carelessly.
rewarding : advantageous; productive.


Synonym of rewarding is profitable