Frederick Muller was a British publishing house. In 1964 and 1965, the company commissioned and published three novelisations based upon serials in Doctor Who: Doctor Who in an Exciting Adventure with the Daleks, Doctor Who and the Zarbi and Doctor Who and the Crusaders.
Here's the covers.
The Dalek cover was also reprinted in grey.
In 1973, Target Books acquired the reprint rights to these three novels and used them to launch its own series of novelisations - a series that would continue to 1994.