In noun phrases lik "ketaab-e man" that a modifier (a noun or an adjective etc) goes after the main noun, the stress is usually on the second word (or the last word if there are more than one word).

In your example it is not that the "ketaab" loses it's stress, but the second word gets the stronger stress.

In cases where a modifiers goes before the noun, the stress may go on the modifier or on the noun based on the type of the modifier:

1. the stress is either on the modifier or the main noun becomes unstressed when the modifier is:

    - a demonstrative adjective e.g.   inketaab, aan mard

    - an interrogative adjective: kodaam ketaab, chand derakht

    - a modifier called sefate bayaani (صفت بیانی): tanha ketaab

2. the stress is on the main noun when the modifier is:

    - a quantifier: do ketaab, dovvomin ketaab

    - a superlative adjective: behtarin ketaab

    - indefinite pronouns like each, every, a little etc : har rooz

Melody of speech in Persian by Taqi Vahidian Kamyar