Abarim Publications' online Biblical Hebrew Dictionary
The particle י (yod) is named after a clenched fist (יד, yad), which it resembles in form but also in function.
Placed after a word, our particle forms a possessive form: something held in one's hand or one's power, and as such it can usually be translated with 'of' or 'my'. The noun ספר (seper) refers to a book, record or account and the term 'my book' is spelled ספרי. A postfixed yod may also form an adjective, which obviously also reflects a kind of possessiveness, and in that case the form ספרי would mean bookish, pertaining to a record, or may even refer to someone from a place called Seper (in which case the form ספרי would mean Seperite). The regular plural of our word would be ספרים but in its semi-genitive form (Hebrew has no real cases) the final ם drops off and again the form ספרי remains, in this case to mean 'books of'.
Placed in front of a verb, our particle renders it a driving force, like an outboard engine, and reflects an ongoing action corresponding to the verb: the verb ספר (sapar) means to create a record, or to relate an account, and the form ספר corresponds to the perfect third person singular masculine: he creates or created a record. The form יספר is the imperfect of that same tense and means he is creating, was creating, will be creating a record. The large majority of Biblical names that start with a J or I (such as Isaac, Israel or Jacob) are these imperfect forms.
The Hebrew letter י (yod) inspired the Greek letter ι (iota), and both appear to have served as symbols of things that are small but nevertheless crucially significant (Matthew 5:18).