Are you talking about the initial query or after they've contacted you?  Some agents only want a query letter, no pages.  If they do take excerpts with the query letter, their website should spell out exactly what they want--and you should do it!

Now, if you're talking about after you've queried and peaked their interest, then it's totally variable what they'll want, and they'll tell you.  For that second contact, 50 pages is pretty common.  However, sometimes an idea will so seize an agent that they'll ask for the entire manuscript right away.  I had that happen twice in my querying process--one of which led to my agent.  Just hope that if they do ask for the whole manuscript, they'll take it electronically.  Shipping's a bitch.

20 Master Plots: And How to Build Them  by Ronald Tobias

Should be available at all your favorite online book retailers.  Covers the biggest archetypes in stories and has a sister book by Victoria Schmidt called 45 Master Characters.  Neat reading, particularly if you're into the myth and archetype thing.  If you need more specific ideas, pulling up those books on Amazon will lead you to a gazillion others via their "suggestions/similar" feature that actually list out very specific plot and subplot points.  I've never read any of those, but you might be able to find something useful.  Good luck!

I forwarded that link to some of my author pals.  I can't reveal their names, but you've probably heard of a lot of them.  They proceeded to flood my inbox with title suggestions for a sequel to Big Spankable Asses. It was highly inappropriate. 

OMG.  Big Spankable Asses.

They published the diary of a 15 year old prostitute, that's certainly not mainstream.

I'm this close to making a joke about Germany, but I love the Germans too much.   

I was able to deduct stuff before I was published.  Conferences in particular qualify as you educating yourself for a potential career, and the travel expenses go along with those.  I think there were some other things...seriously talk to a tax professional.  I think you'll be surprised at what you can do, even not published.  And once you are published, yeah...the deductions are crazy.  I was astonished at how much I was able to write off this last year.  The downside, sadly, is that being self-employed means having to pay Social Security twice as part of the self-employment tax.   Being published will also mean paying quarterly taxes on your own since the publisher doesn't deduct them.  Seriously...talking to a tax person.  Good luck!

wow, I had no idea authors had so much time on their hands.  all that writing to do and whatnot.....

Heh, well, I think it depends on the author.   I almost had a nervous breakdown a couple weeks ago with my workload, and I suspect I have it light compared to Jim!!

This advice is right on.  Focus your efforts on selling the first book, but do mention that you have plans for a series.  If an agent's interested, he or she will ask you for more details about subsequent books later--so definitely don't submit in-depth stuff for a bunch of books right away.  Just make that first sound as awesome as possible.  Good luck!