"Harrington on Hold'em Vol. 1" is a seminal poker book in the field of Texas Hold’em instructional literature, and it details many strategies like the squeeze play, longball and smallball, firing the third bullet, and other intricacies of the game.  I would dub it a must-read for any aspiring student of the game.  Harrington focuses on pre-flop hand selection, including the influence of position of which hands to play and what hands to play in the tournament's three stages, early, middle, and late.  His conservative hand choices don't belie his reputation as a "rock", indeed, he endorses raising the standard 3.5-5 X the BB when dealt the large pocket pairs, but being sparing in hand selection when under the gun. 

He does, however, endorse the occasional Negraneau-esque raise with a suited connector from early position, to insert an element of unpredictability into one's poker game and to make you harder to read by observant opponents.  Thus, when your opponent sees a 3 X BB raise from early position, he doesn’t know whether he is facing a six-seven suited or J-J, etc.  Also intriguing is Harrington's devaluation of the Ace with a weaker kicker, due to extreme problems with domination when an Ace flops.  He metaphorically describes the dilemma as "plummeting off a slope" in reference to the descending value of the K-Q-J, and 10-kickers of the ace in ace-kicker...He also intriguingly recommends the discarding of such "problem" hands as K-J and K-Q and Q-J from early position, since if you are raised from late position you could easily be dominated and are at most a slight favorite to win the hand.  Finally, Harrington points out that the "8" is pretty much the "cutoff" kicker for Aces in terms of their playability from the professional perspective, with A-7 and smaller being fundamentally too risky and susceptible to domination by a stronger Ace.  Also, A-2, and A-3 have the potential to make hidden wheel straights on a rag flop, potentially winning the entire stack of someone with a set or high pocket pair.  These are the basic hand selection guidelines that the conservative Harrington endorses in his seminal work. 

Harrington also explores the nature of the texture of the flop as an important element in deciding whether to bluff, and whether your hand is any good.  Also, coordinated flops like 7-8-9 double suited are particularly dangerous for the high pairs and thus should be bet aggressively to shut out drawing hands like A-10 or Q-J of that suit.  Harrington, a risk analyst, among other things by profession, also focuses prominently on pot odds as the key factor in making decisions, and has a cogent introduction to determining hand probabilities based on the number of outs, and using this information to know how much to bet to out price flush draws for example.  A flush draw is about 36% to complete by the river, or a little better than 2:1 against, so you should bet enough to give your opponent no more than 2:1 pot odds to call; a bet of 2/3 the pot is usually enough to accomplish this out pricing. 

To practice such intricate mathematical concepts, Harrington also has an extensive "Problems" section that includes sample hand analysis and how Harrington himself would have played particular hole cards, etc. Overall, I believe that this book is invaluable to the aspiring player and is a must-buy, in addition to "Harrington on Hold'em Vol. 2" and "Harrington on Hold'em Vol. 3". and the two works on Cash Games, Vol.s I and II.