You can look it up on the post office website. 

There is a chart in the PBS Help Center.  Current Postal Rates

(I print the chart out to use, even take it with me to use with the APC machines.)

one to three ounces is $1.95. As long as you're sure it's under 3 oz.

Under 7 oz. is actually cheaper to use the First Class rate.

I recommend finding a way to weigh it.   If you're going to apply postage at home, you really need a scale.

I would suggest investing in a postal scale! I just recently bought one & it's great! I wish I'd gotten one sooner.

Forever stamps, worth .46 now?

   .46
    X4
$1.84

What rate are you using, is that enough postage for your weight?
 

The correct postage would have been $1.95 according to the post office postage calcutator.  I small package cannot be sent letter rate. Even a very thin book should fall under the large envelope or small package rate.  Maybe if it's thin enough it won't have an issue. 

It came up as .85 for an envelope containing a rigit object. But I think that just means a standard sized envelope.

Over 1/4" thick would take it out of the envelope category.

(One of my jobs at work is dealing with mailing-catalogs, price lists, brochures etc..., )

I found a pretty decent digital kitchen scale in Target for about $20. I can use it to measure my portions when I'm attempting to watch my weight and use it to weigh my books and it's accurate to tenths of ounces. For that size book you'd probably be good with about 5 stamps. I find a standard length MMPB takes about 6 regular Forever, or 46-cent stamps.