13. A non-transitory machine readable storage device tangibly storing a program of machine-readable instructions executable by a computer-based machine, which instructions, when performed by the computer-based machine, cause the computer-based machine to perform operations comprising: receiving, over a network, via a network interface, a digital location identifier for navigation to a shared reading location with an associated annotation, said digital location identifier specifying the shared reading location within a version of the electronic reference document and does not include the version of the electronic reference document; upon receipt of a selection to navigate to the shared reading location, determining if a user has sufficient rights to use a copy of the electronic reference document referenced by the digital location identifier, and, if the user does not have sufficient rights, (i) providing a prompt with an option for the user to purchase said rights, (ii) responsive to confirmation of the purchase of the rights to use the copy of the electronic reference document, receiving the copy of the electronic reference document, and (iii) storing the copy of the electronic reference document in the digital memory; and upon successful confirmation of the user's rights to use the copy of the electronic reference document and responsive to the selection to navigate to the shared reading location (i) determining that the content referenced by the digital location identifier exists in the copy of the reference document, (ii) displaying at least a portion of the copy of the electronic reference document at the shared reading location and the associated annotation within the copy of the electronic reference document at the shared reading location.
16. The non-transitory machine-readable storage device of claim 13 , wherein the annotation comprises at least one of: one or more words, one or more numerals, a mathematical formula, a graphical image, a link to a location, a universal resource locator (URL) address, a script, and highlighting of selected text.