The search function is your friend...in fact, google is holding a party and you're invited!
Like the good men said, lots of info on line. Also, check the Park Cities Tactical (Cult of the P7 board) site and you will find a wealth of information. P7 developed in the 70s to compete with Walter (P5) and Sig (P6 or P225 more recently) for German national police force contract. Primary contract requirements were to be 9mm (because of standard at the time with U.S. adopting 9mm with adaptation of the Beretta 92 to replace the venerable 45 cal), get quick into action (minimal time for safeties,etc.) due to terrorist massacre at '72 Munich Summer Olympics. Sig proposed DA/SA model with decocker to solve getting into action quickly while H&K devised the unique squeeze-cocking mechanism for the grip's frontstrap. This allowed a very sweet SA trigger to be installed and the combination of these two aspects made the P7 one of the quickest semiautos ever to get into action. The combination of a fixed barrel and utilizing propellant gases to in a gas port/chamber under the barrel to retard/slow slide movement rearward after shot reduces felt recoil and makes it extremely accurate. In addition, a low bore axis allowed H&K to have the barrel be low relative to shooter's hand. This pistol design won the contract and as a result, many of these guns ended up on shelves of various German armories and have come over in several large shipments to U.S. importers over the years (PW Arms - Redmond, WA and more recently the HK Trussville, AL shipments in 2007/2008, and most recently HK Columbus, GA shipments). The use of the propellant gases causes the trigger guard to become very hot after shooting several mags of ammo in quick succession. In the words of Massood Ayoob, a very knowledgeable CCR instructor(paraphrasing here), "the great thing about the P7 is it is easy to shoot, the not-so-great thing about the P7 is that it is easy to shoot". As a result, the P7 was criticized for resulting in accidental discharges in both Germany and the U.S. The first P7s (such as your one) have a European grip heel mag release; however, H&K determined in the early 80s that in order to compete for the U.S. market that it needed to make improvements such as installing a plastic heat shield over the trigger guard to protect against heat, installation of a mag release to the more U.S. accepted location behind the trigger, as well as a loaded chamber indicator and ability to easily remove the firing pin assembly, and renamed this version the P7M8 - which has become the more popular and expensive variation to obtain in the U.S. since that time. In addition, H&K developed other models of this pistol family to include a 13 shot version with a double-stack magazine design called the P7M13, a short-lived P7M10 variation for the 40S&W round with an extremely thick slide making it one of the heaviest steel gun designs in recent memory, as well as the more rare P7K3 version (3 Kaliber interchangeable barrel design between 22, 32, and 380) which commands the highest premiums. The above notwithstanding, I am a fan of the P7 design for both range and self defense applications - have an M8, M13, and soon a P7-PSP. Hope this helps.