Broadly speaking, general-purpose fonts are divided into two families.
Serif fonts. These are fonts like Times New Roman or Garamond which have serifs. These are little lines or strokes at the termination of letters. Originally these were used to mimic pen strokes. They still evoke the feel of careful calligraphy (not just sloppy handwriting) and you often find that the weighting of the letters mimics the use of a broad pen. Nobody is going to mistake a serif font for handwriting but that is the origin and the rationale.
San-serif fonts. These are fonts like Verdana and Arial which do not have serifs. Simples.
I'd encourage anyone with the slightest interest in this sort of pernickety topic to go and read all about different fonts. There's a lot of depth and it's a fascinating area. For this journal, I'll be sticking with the basics: It is a truth, universally acknowledged, that a reader will find a block of serif text easier to read. The bulk of the text should therefore always be done in a serif font. To give contrast you should pair this with another font for things like chapter headings. You can pair a serif font with a sans serif, and this is a very usual approach. There are lots of lists of suggested pairings online and you can browse away to your heart's content. Even the pickiest person is guaranteed to find some which light up their hearts with joy.
Next problem: Fonts ain't always free. Making a well-balanced font is a huge undertaking and some people have spent years on a single font. Some people do this for a job. The point at which you're making and selling a book is the point at which you will generally find that the free personal use licence on your favourite font reaches its limit. You can choose to buy the font you want (anywhere from £10 up to £100s) or you can find one which does have free commercial use.
Enter Google Fonts. At the moment they offer 991 font families with a free commercial use licence. Some are very 'full' families, offering a dozen or so different weights, others simply offer a bold and italic in addition to the normal script. There are a number of free font websites out there but I would recommend caution. They are not always clear on the limits of the licences of the fonts they offer. If you do find a font on one of these that you absolutely must have, take the time to track down the creator's website and find the actual licence. You may discover that the badge declaring 'free commercial use' on the free font site applies only to use of the font when it's flattened into print designs, not when it's embedded in a book. This is a complex area and not one to play fast and loose with other people's stuff.
In the end, I settled on a nice pairing of Montserrat for heading, and a lovely serif font called Cardo for the body.
Rather fetching don't you think?
That's more than enough on fonts from me but it's an amazingly interesting area, and I'd recommend anyone who is serious about typesetting their own book spends a few days reading about it.