The Symbolist movement is usually seen as a largely literary affair, confined to local little islands of thought. Lucie-Smith's goal with this book is to restore Symbolism in painting to its proper place within the arts in Europe in general and, more importantly, to remind us of its location on the trajectory towards what is called Modernism.
Symbolism was the piece linking Romanticism and all that crap to the emergence of artists like Picasso (whose early work was exemplary Symbolism). Movements in art didn't simply emerge miraculously from the divine slime, but rather evolved out of other movements just as they vanished into them.
Symbolism was an expression of displeasure with the mores of the time through ambiguity, symbolism (duh), the reactive summoning power of the image, and synthesis as opposed to analysis. Art was meant to parallel the world, not be of it, the Symbolists might argue.
There's a lot of ground covered here and Lucie-Smith does a fine job keeping it concise. A lot of favorites pop up and some new ones (to me, anyway) are discussed. Moreau, Redon, Gauguin, Rops, Klimt, Munch--scads of others are included. He takes a little more time with the ones he deems crucial, like Munch who he sees as being the main bridge to Modernism.
Tons of plates, too, which is always nice.
Hooray for dark and terrifying artistic equivocation! Feel the terror!