You've got some excellent advice already listed here! I agree with ALL of it.
Best website in addition to mousesavers.com is:
www.disboards.com
They will give you lots of touring scenarios and advice on that site and even though I am a FL resident I always refer to them to get the best deals. There is a free or cheap section listed with all sorts of nice hints.
Get these free maps from Disney mailed to you - they are good keepsake.
http://disneyworld.disney.go.com/wdw/myVacation/customize...
Also, if you haven't been there before, request a pin to wear around the park, they give you a First Time Visitor button or a Birthday button or whatever, just if you ask them at the gate.
As others mentioned, entertainment books don't give you discounts for any of the theme parks so that won't help you.
Yes, Harry Potter lines are overwhelming - bound to settle down by the time you get there next year. There is plenty to do at Islands of Adventure in addition to the Harry Potter portion of it.
Discovery Cove has swimming with the dolphins - I haven't heard of swimming with the manatees except at Crystal River which is a bit of a drive from Orlando.
Have a fun time and be prepared to spend - just try to keep repeating the mantra that you are on vacation so you have to part with the money. I am a bargain shopper but when it comes to Disney you can only save so much with them...
If you go to your local library, check out one of the Orlando travel books and plan your trip that way too. Sometimes it's easier on the eyes to read a book, rather than try to read something online from a website.
Bring your own water bottles is great advice. Bring a stroller or be prepared to rent one if you have kids because everyone gets tired and grumpy after too much walking, especially in the heat. You can always go back to to hotel or to another restaurant, swim, take a break in the afternoon - and then head back to the parks rather than eat there too, just to save money.
Depending on how busy the WDW park is, sometimes as a token of appreciation for putting up with the crowds they will hand everyone a 20% off voucher to use at their restaurants when you walk into the park. Watch for those - money off coupons are always a nice unexpected perk. The message boards post prices for menus,(and helpful reviews too) so read up and be prepared cost-wise because else it will be sticker shock.
If money is tight though, buy souvenirs at Publix or the local Walgreens or tourist store even, everything is cheaper there and great selection. Have fun!