The novel is probably the best expression a writer can give to examining an idea, or group of ideas. The best ones will have sub-texts and/or subplots as well as the main over-arching story. They vary from 250 to several thousand pages, and that's without looking at Trilogies, series etc. Like most media (e.g. pop songs, films), novels have got longer in more recent times, but that doesn't necessarily mean they are better..... The main issue with older writing is that modern sensibilities can be offended, like in 50s novels (and films!) the females main role is screaming and being saved by the male hero. Being S.F. there are exceptions, but even in Science Fiction many female writers felt they had to pretend to be male to be successful (James Tiptree Jr. for example).
Here is a list of suggested reading, split into categories:-
First Contact/Aliens.
Dragons Egg - Robert L Forward. Classic "hard" SF (i.e. its got science in it!), outstanding aliens, sequel is Starquake.
Tilting the Balance - Harry Turtledove. The first in a series (currently 7 books) about aliens arriving during the 2nd World War. Turtledove is an historian and it shows in the details. He specialises in Alternative History, books which say what if? The essence of Sci-Fi.
Little Fuzzy - H Beam Piper. A Classic.
The Hitch-Hikers Guide to the Galaxy - Douglas Adams. The first book of a trilogy in 5 parts, the radio plays will lose you a long car journey very easily.
Cyberpunk (hi tech near future stuff)
Neuromancer - William Gibson. The Father of the sub-genre. Excellent story & writing which is what the point should really be. He still has my favourite book title "Mona Lisa Overdrive".
Snowcrash - Neal Stephenson.
Altered Carbon - Richard Morgan. Not strictly Cyberpunk, but utilises many of the genre's marks, superb, amazing it's his first novel.
Other Authors for Cyberpunk - Tricia Sullivan, Pat Cadigan, Mel Odom, Bruce Sterling.
New to S.F.
These books give cross section over the science fiction arena, the books listed in the other catergories would help too; it's a big universe out there!
Eon - Greg Bear. A modern classic.
Friday - Robert Heinlein. This book invented the girls kick ass sub genre, (seen more on TV shows, Buffy, Dark Angel, Xena).
The Jonah Kit - Ian Watson. There was a phase in the 80s/90s where whales were used a lot in SF, Star Trek IV, The Voyage Home anyone? This is (much) better.
Memoirs of a Spacewoman - Naomi Mitchinson. One of the strange things about SF is it is hard to define, many mainstream authors have written SF but would deny it - The Da Vinci Code? Anyway, this Lady was in her late 80s/90s when she wrote this, and puts younger writers to shame.
Enders Game - Orson Scott Card. Another Classic.
Stark - Ben Elton. Did I mention about mainstream authors writing SF?
Dragonflight - Anne McCaffrey. Welcome to Pern. First in the recommended reading sequence.
To Your Scattered Bodies Go - Philip Jose Farmer. One of the more "lyrical" authors. First of the Riverworld series.
Foundation - Isaac Asimov. First of what became a galaxy spinning series of books.
Rendezvous with Rama - Arthur C Clarke. First of what is currently 4 books.
The many Coloured Lands - Julian May. Time travel to prehistoric earth, first of the saga of the exiles.
Galactic Colonisation & Future Earths
The Reality Dysfunction - Peter F. Hamilton. I actually prefer his lighter books (Mindstar Rising etc.) but no doubting the galactic future built with the trilogy started with this novel.
Orbitsville - Bob Shaw.
Hidden Empire - Kevin J Anderson. More famous for writing film/TV tie-in books, this is the first of an outstanding series about Mankind out in the galaxy. It is set to be seven, the sixth is due out in Summer, 7th in autumn (fall) 2008.
Consider Phlebas _ Iain M Banks. The first of the Culture novels, Banks is the best at giving you an idea of what a galactic civilisation would actually feel like.
Exploring the Solar system.
Ben Bova probably owns this area, as he's written a book for each planet (and each book is called Mars, Mercury etc.), and some other bits of the Solar system too.
Sam Gunn, Unlimited - Ben Bova.
Red Mars - Kim Stanley Robinson. The first of his Mars Trilogy (Blue and Green being the other two), just outstanding future history about how we colonise Mars.
Stardance - Spider & Jean Robinson.
Fantasy
I can't suggest much of the current fantasy market as I don't read very much of it; try not to let the covers the books have put you off! Most modern fantasy is medieval based/swords and sorcery/ vampires etc. which generally doesn't interest me. There are always exceptions but they have to get my attention. Some Fantasy I would recommend are the following:-
Lord Foul's Bane - Stephen Donaldson. The first of the saga of Thomas Covenant, not the lightest read on the planet, but engrossing. A New trilogy is gradually being published.
Little, Big - John Crowley. Just let this book wash you away to another land.
The Warlock in Spite of Himself - Christopher Stasheff. Fantasy doesn't have to be heavy going, some people would this this book in a "science-fantasy" sub category, just enjoy a light read. First of several books about Rob Gallowglass.
Stonor Eagles - William Horwood. The bird of prey equivalent to Watership Down, but much better than that would suggest.
The Runestaff - Michael Moorcock. This author is up with Isaac Asimov for quantity of output, but Moorcock is much more in an experimental/ Fantasy vein rather than keeping the science right.
The Colour of Magic - Terry Pratchett. The first of the Discworld series. If you don't know about Pratchett, have you been on Planet Earth recently?
Perdido Street Station - China Mielville. Almost single handedly creating a new sub-genre within fantasy, this book explores the city of New Crobuzon. Blurs the "boundary" with SF as well, no bad thing.
Split Infinity - Piers Anthony. Another light fantasy, or so called science fantasy books. First of Trilogy.