D&D Beyond is the site that sells the official electronic versions of D&D. For some gamers, the electronic version of the rulebooks are the starting point for their Dungeons & Dragons experiences, but others prefer physical books. If you want both, you have to pay for both.
In the last few week D&D Beyond
The most common code is;
This coupon gets 25% off all the sourcebooks.
The BELEGENDARY25 code is reported as working on the Legendary Bundle as well and represents a pretty good deal.
These coupons could stop working at any time.