The RHS encyclopedias are good. If you can find an old readers digest encyclopedia of gardening thats good too.
If you have plenty of time on cold winter nights, then the history of gardening, from William Robinson, to Gertrude Jekyll, to Marjory Fish, Vita Sackville West, anything by Beth Chatto, Christopher LLoyd. I also enjoyed the books by the plant hunters like Frank Kingdom Ward, Roy Lancaster 's book of Travels in China.
The DVD set by Geoff Hamilton is still available, if you want basic gardening techniques.
If you join the RHS you get a magazine every month which I keep for reference. You can also take part in the seed distribution each year, which can throw up some rarities for you to try.