A book (a 'trick' in other card games) is the fundamental unit of Spades. Four cards are played, one from each player, and whoever plays the highest wins the book. There are 13 books per hand. Everything about scoring is built on how many books your team takes vs. how many you bid.
How a book is played
One player leads by playing any card from their hand (subject to the spades-broken rule). The three other players then play one card each in clockwise order. Whoever plays the highest card by JJDD hierarchy wins the book.
The winner of the book leads the next one. Play continues until all 13 cards have been played and 13 books have been taken.
The follow-suit rule
If a heart is led, you must play a heart if you have one. Same for diamonds and clubs. Only if you have no cards of the led suit can you play something else — a spade to cut and win the book, or a discard from another suit to throw off a card you don't want.
This is the rule that makes JJDD strategy possible. Being short in a suit is a weapon: it lets you cut. Being long in a suit is a liability if it's not spades: you'll get stuck following suit while your partner needs you to cut.
Books and your bid
At the start of the hand your team bids a number — the number of books you promise to take. Take exactly that number, or more, and you make your bid (10 points per bid book, +1 per extra as a bag).
Take fewer and you get set (−10 per bid book). This is why every book matters — even the ones that seem meaningless in the middle of a hand often decide whether a team makes or breaks.
