Rodale's Illustrated Encyclopedia of Herbs is an excellent reference book, giving detailed information on herbs that can be grown in the garden, with details of cosmetic, culinary and medicinal uses.
If I want to know real in-depth information about a particular plant, I read Christopher Lloyd's books. Many books on garden plants seem to trot out the same basic information. Christopher Lloyd seems to have grown almost everything and can write from experience about which plants are worth growing and which are often disappointing. He can be very funny, too. I particularly like The Well-tempered Garden, Colour for Adventurous Gardeners, and Christopher Lloyd's Garden Flowers.
"What a blessing it is to love books. Everybody must love something, and I know of no objects of love that give such substantial and unfailing returns as books and a garden."
- Elizabeth von Arnim
Recent reading
Gardening books
Poetry books
Meaningful miscellany
A
Gentle Plea for Chaos
Mirabel Osler
The
Complete Gardener
Monty Don