I agree on the owners manual. If you really study this book you will develop a good feel for what the truck needs to keep it healthy. There may be a Chilton's type book for your truck that probably just restates the information in the owners manual, but may bring out some subtleties that the owners manual doesn't focus on.
It's good to make a list of all the items on the truck that need periodic maintenance so that you can make sure that things don't get overlooked.
Keep an eye on the Speedco types. They mean well but they can screw a lot of things up. Just watch them.
The following are just some basic items, most of which are obvious, but you may see something you hadn't thought of:
It's good to know roughly where all of the grease fittings are so that you can make sure that the oil change guy (unless you do it yourself) isn't overlooking anything. If you grease the chassis yourself, jack up the front prior to lubricating the front end and move the wheels to the right and left. Chock the wheels and release the brakes before greasing the brake grease fittings. These steps are not vital, but some people think they allow the grease to get into the nooks and crannies better.
Don't mix coolant types.
Change your oil, oil filter and fuel filters on a schedule. Check and see if you have an in-line fuel filter in addition to your spin on fuel filter. If you have an in-line filter buy a bunch of them because the oil change place almost certainly will not have these filters. This may just be a CAT 3126 thing, but mine has an in-line filter and no one ever has them at truck stops. You can buy them at NAPA.
Check your air filter periodically. Keep your window washer topped off (I've been using the yellow Rain-X washer fluid and really like it). Lubricate the linkages in your windshield wipers from time to time. Monitor your power steering, transmission, rear end, and hub fluids. Check your tire pressure regularly.
There are some aerosol spray-on dry grease products that are great for door locks and hinges and quieting squeaks.
Put dielectric grease on electrical connections to prevent corrosion. Consider LED lights where possible when you have bulbs burn out. Know all of the different fuses that your truck uses and have spares. Keep a spare flasher.
Have your truck pressure washed after driving in icy conditions where de-icing chemicals are being used. Wax your hood and the tops of your fenders periodically or the whole truck if you are in the mood.
Put some torx screwdrivers in your tool box. You'll need one sooner or later and you'll be glad you have it. Get some Loc-Tite to keep screws and bolts from getting loose from vibration. Get some cable ties/zip ties in different sizes (they have a million uses).
Throw some safety glasses and an orange vest in your truck. You'll need them at many shippers and they are handy for other things as well.
Sand and seal the wood floor inside the cargo box as needed. Maybe once a year.
Have the bottom of the cargo box sandblasted and painted if you begin to get a lot of rust underneath.
Ask a mechanic to show you how to adjust your clutch (if you have a manual transmission). You may not ever want to do it yourself, but in an emergency it's good to know how to do it if, for example, you are climbing a steep hill and your clutch starts slipping so much that you can't get up the hill. Once you know how to do it, it's very easy to do.
Those are just a few things that come to mind that are good to pay attention to.
