They're valid, though clumsy in the examples, but then the suggested alternatives are also clumsy in the examples.
In each case, meaning is clear; "other than [noun]" is a noun-phrase that means those things that are not what the noun indicates.
Two examples I can find are:
From the moment of our introduction, he talked so comprehensively and quickly that I felt no need to ask other than occasional questions. — Anthony Hlynka, The Honourable Member for Vegreville
In fact, many women did not easily ask other than close kin for support. — Franz von Benda-Beckmann & Keebet von Benda-Beckmann, Social Security Between Past and Future
These are fairer examples of the form. There would be nothing to gain in making them "ask questions other than…" or "people other than…".
The two examples in the question I would rephrase, but I would rephrase them entirely, e.g. "He doesn't just read mathematical books".