These participles, like all participles, can be paraphrased with который-clauses. Unlike the other participles they are rarely used, even in written style.
The formation of the present passive participle (e.g. читђемый) differs from the formation of the past passive participle (e.g. прочќтанный) as follows:
(1) It is formed on the present (i.e. non-past) stem of the verb rather than on the past stem. In fact, it looks just like the non-past мы-form (first person plural) plus the adjective ending: читђем-ый.
(2) It is formed from Imperfective verbs rather than from Perfective verbs, and therefore has the meaning of on-going action or repeated actions rather than the single, completed action typical of a Perfective verb. Compare:
present passive (Imperfective):
  кнќга, читђемая нђми the book being read by us
    кнќга, котћрую мІ читђем the book (which) we're reading
past passive (Perfective):
  кнќга, прочќтанная нђми the book read by us
    кнќга, котћрую мІ прочитђли the book (which) we've read