It goes a bit like this:
okumak "to read" - infinitive
okuma "reading" - a noun indicating the action of reading.
okuman "your reading" - the fact that you read, or your action of reading
okuman lazım "your reading is necessary" = it is necessary that you read.
o kitabı okuman lazım "your reading that book is necessary" = it is necessary that you read that book = you need to read that book.
To supply mizinamo: the personal pronoun connected to the gerund would be in the genitive form, in this case 'Senin o kitabı okuman lazım'. If it were he/she the sentence should be 'Onun o kitabı okuması lazım.'
İn other cases where the personal pronoun is the subject of a sentence, it should be followed by a comma, e.g. 'O, o kitabı okur. (she reads that book.)' or if it is not a particular book just 'O, kitabı okur. (She reads the book.)'