""The Bogleheads' Guide to Investing 2nd Edition",
by Mel Lindauer (Author), Taylor Larimore (Author), Michael LeBoeuf (Author), John C. Bogle (Foreword)"

I like the "little Book of..." series, one of which is by John Bogle - Little Book of Common Sense investing which sets out the benefits to investing through index funds.

Others in the series I really like are

Little Book that beats the Market by Joel Greenblatt (it has a slightly newer version out- '...still beats the market', and a third edition is supposedly in the works but the release date on Amazon keeps moving),

Little Book that builds Wealth by Pat Dorsey

Little Book of Valuation by Aswath Damodoran

Honourable mentions to James Montier's book in the series on behavioural investing and Christopher Browne on Value investing.

Others have mentioned books like One up on wall St and Warren's letters which I love as both are fantastic reads.