The Companion Expansion by Barrataria Games didn't get the same level of hype as the B/X Companion, but it covers much of the same ground. I have not looked at them together to see how they cover similar topics, but they seem very compatible.
For new races there are gnomes and wild-wood (sylvan) elves, half-orcs, half-ogres and half-elves as race-classes. Wood elves share the same spell lists as do druids and gnomes share a list with Illusionists and bards. All for the B/X system
It also has a Bard, Druid, and Illusionist Classes that are roughly equal with the AD&D versions. But the real feature of this book is the expanded spell list. If you are looking to extend your illusionist, druid or bards a bit more with more spells then this is a good way to do it.
While this book does not have the feel of the first Basic and Expert books, it does have a ton of useful information and all of it (well all but about 0.5%) is 100% OGL.
I consider this book a forgotten treasure.