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 suffixed 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.
Note that the familiar phrase "the laying on of hands" uses the verb סמך (samak) for "laying on." From this verb comes the noun סמך (samekh), which is the name of the fifteenth letter: the ס.