Reddit reviews Mophorn Paper Cutter 12Inch A4 Commercial Heavy Duty Paper Cutter 300 Sheets 45HRC Hardness Stack Cutter Metal Base Desktop Stack Cutter for Home Office (A4, 12Inch/45HRC Blade Hardness)
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▇ LARGE CUTTING SIZE--300 sheets of 20LB bond paper high or approx 1.5"(for 70g A4 Paper)▇ AUTOMATIC SAFETY LOCK-The blade edge is made of high speed steel (HSS) with the hardness grade of 45HRC.Automatic safety blade lock prevents cutting until released. The blade is re-sharpen-able and replaceable▇ ACCURATE PAPER HOLDING--Machined solid aluminum side gauge with measurement scales in standard inches (1/32" resolution) and metric (1 mm resolution),this commercial-quality paper cutter is designed to last and perform▇ SOLID ALUMINUM ALLOY BACK SUPPORT--Equipped with the optional back support/extension table. The back support/extension table extends the back side of the cutter and provides a more well balanced and stable platform for the cutter▇ WIDE RANGE OF USES--The paper trimmer cutter is perfectly suited for use offices, schools, churches, businesses, printing houses as well as professional individual users
You didn't say so in the text, but I'm going to assume that you cut away the cover first and are just trying to remove the part that holds the pages bound together.
When bookbinders are putting those bindings in place, they also have to cut the pages square to make them line up. For this purpose, they use heavy-duty varieties of paper cutters, which cut the pages neatly and without mess.
Honestly, I'd recommend you visit a bookbinder, and just ask them to unbind those books for you; they have the tools and the skill. A circular saw cut (or any other saw cut, really) is never going to be clean unless maybe if you glued the pages together first, but that would defeat the purpose of what you're trying to accomplish.