First, a correction to my last post.  I discovered that skill books don't disappear once your Sim has read them, although it can seem that way.  When your Sim finishes reading a book, it goes into their personal backpack (the panel formerly known as Inventory).  So it LOOKS like it disappears, but it's there, honest! 

This is one of the things I grapple with, after having played Sims 2 for so long.  Unlike Sims 2, where the inventory was almost completely under your control, your Sims stick things into their own backpack fairly often.  Pretty much anything they pick up could end up in their backpack. 

And I'm still a little fuzzy on the programming logic behind the backpack.  I know that if you stick a plate of food in there, it will eventually go bad.  However, when your Sims catch fish, those seem to have an indefinite lifespan in there.  As do the groceries they purchase from the grocery store, and the fruits and vegetables they harvest.

I did some cleanup for one of my families, and discovered that I had a total of three Level 1 cookbooks and two Level 2 cookbooks.  (Along with a huge collection of rocks, fish, old newspapers, what have you.)

You can tell your Sim to put the books back on the shelf, or you can click and drag them onto the bookshelf directly from your Sim's backpack.  You can get rid of extra books in two ways: your Sim can put it into their inventory and take it to the bookstore, where they can sell the book back.  Or you can go to their backpack and just drag and drop the book onto the "click here to sell" square.

I did some more research into the discounted bookstore recipe prices, for a Sim with the Bookstore Bargainer Lifetime Happiness Reward:

Level 1 recipes (Ratatouille, etc) 19 Simoleons.
Level 2 recipes (Fish and Chips, etc) 38 Simoleons.
Level 3 recipes (Cookies, etc) 75 Simoleons.
Level 4 recipes (Parfait, etc) 150 Simoleons.
Level 5 recipe (Ambrosia) 9,000 Simoleons.

There is only one Level 5 recipe.  My understanding is that eating Ambrosia sets back your Sim's life clock, similar to the milk of the Cow Plant in Sims 2.  I haven't tested that yet, because my Sims haven't accumulated that much money!  I'm saving up for a remodel, and after scrimping so far I've been able to get them to build up about 2,000 Simoleons.  (See my last post for a discussion on the economics in Sims 3.)

One of my Sims has hit Handiness (formerly Mechanical) Level 3, which allows him to dispense upgrades to a number of household appliances and fixtures.  Pretty neat!  So far he has made the toilet, shower, and kitchen stove all self cleaning.  This process takes a Sim afternoon, and involves the use of a hammer.  In the case of the stove upgrade, it also involved the appearance of sparkles.  I'm a little sad that the toilet didn't sparkle when he upgraded it with a hammer, but oh well.