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THE TWO MONOS
者・物
There are two kanji which have a kun reading of mono. They are easy to confuse; but one refers to a person, and the other refers non-human (mostly inanimate) objects and materials.
者
The first one, 者, refers to people. It generally follows the rule of being read in the on-yomi (SHA) when paired with another on-yomi, and with the kun-yomi (mono) when combined with the kun-yomi of another kanji. Below are some examples.
on-yomi + on-yomi:
作者（さくしゃ） writer; author
学者（がくしゃ） scholar
愚者（ぐしゃ） idiot; fool
kun-yomi + kun-yomi:
働き者（はたらきもの） hard worker
暴れ者（あばれもの） ruffian
愚か者（おろかもの） idiot; fool
What this kanji basically does is personify a quality, action, or state of being. For example, fushō 負傷 is a Sino-Japanese compound that means “sustaining injuries. The verb fushō suru 負傷する= to sustain an injury. When is added to this compound, the result is fushō-sha 負傷者(the injured). Similarly, when you combine the verb namakeru 怠ける (=to be idle; to goof off), with 者, you get 怠け者 namakemono, which means “lazybones” or “slacker.”
物
The second mono kanji, 物, means “thing.” It has two on readings: BUTSU and MOTSU. As you can see from these examples, you can expect this kanji to follow the on + on / kun + kun rule:
on + on
物価（ぶっか） price levels
動物（どうぶつ） animal
書物（しょもつ）books
kun + kun
安物（やすもの） cheap goods
冬物（ふゆもの）winter clothing
夏物（なつもの）summer goods; summer clothing
Also note that although both on readings BUTSU/MOTSU are “common,” BUTSU is somewhat more prevalent. There is also a distinguishable pattern of technical/scientific words favoring BUTSU:
物的（ぶってき） material; physical
物体（ぶったい） substance; object
鉱物（こうぶつ） mineral
物理学（ぶつりがく）physics
The MOTSU reading is most common the end of words, like:
荷物（にもつ） luggage
穀物（こくもつ） grain
進物（しんもつ） present; gift
In fact, words that begin with the MOTSU reading of 物 are quite rare. (One is mossōmeshi 物相飯 = prison rations)
事
To close this section, it would be appropriate to mention another kanji that carries the meaning of “thing.” : 事. The readings of this kanji are JI and koto. It refers to “things”—but abstractions rather than concrete beings and objects. These words should give you a sense of how this kanji is used:
出来事（できごと） event
国事（こくじ） affairs of state
記事（きじ） article
善事（ぜんじ）good deed
悪事（あくじ） evil deed