Regardless of the type (flower or vegetable) or size of your gardens, there are a number of benefits of keeping a log. I have been using a 70 sheet 8x10 notebook spiral notebook for the 15 years since establishing permanent vegetable garden beds. Since I am mid-seventy, that is more pages than needed.
Below is a diagram of my vegetable garden (the paths between rows are omitted). Names and dates of planting are recorded. These reflects early (ex. peas) and later (ex. beans) plantings in the same bed. This data helps determine companion crops (favorable associations) and crop rotation planning. This helps in avoiding planting a heavy feeder like corn in the same location without a year of growing peas or beans. These are nitrogen contributor's - especially if you use "innoculant" to affix nitrogen - (available at Agway in the spring). Other useful data that I could have included is the "days to maturity" listed on the seed packets. You may want to supplement your log by keeping the empty or opened seed packets in a card file. This keeps them handy for staggered planting, for checking maturity days, and as general history.
The second page (below) of each years log has notes on seed purchase costs, crop yield, and general notes (like things to repeat or do differently). Since my writing is not very legible, my notes include a reminder to plant peas 2 weeks earlier due to vines dying in June and to switch one row to "Wando" variety. Also to plant cukes in a sunnier row and thin properly, and to widen paths for easier walking and turning around. These notes aids in future seed and growing choices, etc. This page also has a map of my herbs and flower beds - showing bulb and self-seeding annuals locations. This will help when it is time to dig the gladiola each fall and daffodil bulbs for dividing (if my records are correct, next year will be year 3).
Happy gardening - Mr. C.