I agree with the previous post recommending Stamp Smarter. But I would also recommend the "Scott Identification Guide to U.S. Stamps, Regular Issues 1847-1934, 6th Edition" by the late Charles Micarelli. This is a $40 162-page soft-cover book ... the most recent edition (I think) was released in 2013, the year of Micarelli's death.Added
: Just checked to see if there is a newer edition and found 3 copies of this book offered on Amazon
for $600+! The price on the back of the book is $39.95. At this price, it is a good buy ... needless to say, this is not so at $600.