The rules for copying spells from other spellbooks are rather clear: you need time and money. However, in case you acquire a new spellbook, things are more fogged. Can a wizard use two spellbooks at the same time? If so, you only need time to understand it and can forego the costs, right? Otherwise, what happens if you lose your spellbook? Or, must you use just ONE spellbook, possibly attuning to it?
You must practice the spell until you understand the sounds or gestures required, then transcribe it into your spellbook using your own notation. [..] You can copy a spell from your own spellbook into another book-for example, if you want to make a backup copy of your spellbook. (PHB 114, emphasis mine)
There is no limit on how many spellbooks you can have, the PHB explicitly mentions that you can make a backup. But the spellbook is "yours" because it uses your own notation, and you can only prepare a spell from your own book. Thus if you want to use the spells in a spellbook you found, you will need to spend the time and money listed.
All items that require attunement explicitly state that. The wizard's spellbook is not such an item.