It is true that Eric Drexler introduced molecular assemblers in his Book “Engines of Creation”.
But:
He abandoned this specific idea ages ago in favor of nanofactories which are:
1) easier to reach 2) more efficient and 3) not associated with grey goo.
There are no assemblers mentioned in his technical book “Nanosystems”.
In his recent book “Radical Abundance” he goes to great lengths to debunk the widely spread misbelief that he still sees molecular assemblers as the far goal for atomically precise manufacturing. He was motivated by a certain unpleasant non-trivial incident that damaged the fields reputation and greatly delayed targeted development. In “Radical Abundance” he even introduced a new term for the entire field to reduce confusion. Instead of “molecular nanotechnology” he now talks about “atomically precise manufacturing (APM)”.

Sidenote: Molecular assemblers shouldn’t be confused with non-replicative medical nano-robots or non-replicative utility-fog-units which both are desirable products of future nanofactories. (This wasn’t done in this article — Just some info for readers of this comment)