I am taking this from Aeron Buchanan's Japanese Verbs Chart, so please have a look at the second frame (the green one) if you don't understand what I wrote above.
Is this about conjugating nouns into the stem form or something like that?. To clarify, I am searching for the grammar rule, if you can help to elucidate this matter or put a link to an explanation it would be most helpful.
Note:
The verb stem is the い base or 連用形 base for conjugating verbs, like 書きます,待ちます、読みます、食べます.
EDIT: Interestingly, 買うもの can refer to the things a person buys like 彼女が買うものは... so dictionary form + もの is another useful grammar.
The next item on the chart says もの [noun from verb target], which makes no sense to me. However I can't help to notice that if I add もの to 一段・いちだん verbs it does the same trick for some of them.
Those noun forms are often used in compound words. ~もの can be considered a kind of compound word, too.
読み物 = a thing one reads(noun from verb target) = a reading material
書き物 = a thing one writes(noun from verb target) = writing
読み書き: reading and writing
立ち読み: standing and reading = browse(in a bookstore)
買う in that example is simply an adnominal/attributive clause that is modifying a noun, which is the equivalent to the relative clause in English.
e.g.
彼女が買った本
the book she bought
Do yourself a favor and drop that (as for) stuff. It's helpful as an explanation initially to kinda-sorta see what は does, but one shouldn't make a habit out of using it.