I was referring to the Encyclopedia of the Chess Openings (ECO) A through E. Although only B is currently shown, they have all been books.

There is the Small Encyclopedia of Chess Openings. That is available as a book or to be downloaded.

The Encylopedia of Chess Combinations is available on CD. However, I was talking about ECO A through E.

The Encyclopedia of Chess Openings is a multi-volume version of MCO. It's not very readable as there is no text. I recall that one GM said he developed his repertoire by finding mistakes in ECO! Aside from that, I think it remains a reference to many players. I'm surprised it has not entered the digital age.

https://sahovski.com/Encyclopaedias-c25038250

Martyn Kravtsiv wrote Practical Chess Openings. It is available for tablet computers at Gambit.chess. I find it very readable but more of an introduction or overview, whereas ECO comes close to being exhaustive.

Perhaps ECO and PCO work well together.