That rifle is a British Short, Magazine, Lee-Enfield (SMLE), Mark III; it was made in 1909. The ER stands for Eduardus Rex (King Edward VII), who was on the throne at the time. It should also say Sht L.E. and III abbreviating the above info. It was made by the Birmingham Small Arms Co., in Birmingham, England.
It is an interesting gun historically and may well have seen action in WWI.
It was originally chambered for the .303 British cartridge, but it would be best to make sure before firing it that there has been no change and that the headspace is OK. .303 British ammunition is available in most gun stores and can be ordered shipped by express from surplus gun and ammunition suppliers.
Value is not terribly high, though if it is good shape and all original (unlikely after so many years), it could bring $400 or so. An average price is closer to $100-150.
Jim
