On twitter they say "ЧИТАЕМЫХ" and "ЧИТАТЕЛЯ" Is this pattern productive starting with the root of the verb (ЧИТА) and applicable to other verbs?
In some cases this pattern is productive, for example:
- "интервьюер" - "интервьюируемый" (interviewer - interviewee)
- "экзаменатор" - "экзаменуемый" (examinator - examinee)
- "испытатель - испытуемый" (tester - testee)
In this case, indeed, both words are nouns, the last one formed by changing the part of speech from passive participle to noun.
However, this rule cannot be used in some situations:
when the meaning is not "the person who is being exposed to an action" :
we can't say "спасатель - спасаемый" (this one is unnatural), "работодатель - работоданный" (this is simply incorrect)...
when we are not talking about a person: this happens in your example. A book can be read (for example : читаемая мной книга) and it will be a passive participle, but there is no word for a person that is made to read by his angry mother =)
After a short survey suggested by Anixx, I have found out that, indeed, the passive participle "спасаемый" can be used as a noun (mostly in The Bible). However, I have a feeling that the semantics is slightly different : those nouns derived from passive participle can sometimes mean "somebody who can be (...insert the indefinite aspect of passive participle here)". So I have a feeling that "спасаемый" could mean "the one who still can be saved"...
The noun "спасаемый" really exists, as shown in Quassnoi's comment.