In reply to greatshearwater:
Went there two years running about 20 years ago, so some of my info may be a bit old.
Think of it as the Alps but with no huts, no people, (no queues on the mountain routes), no mobile signal and no easy rescue like there is in the Alps. Camped out amongst it is just awesome (and scary; see 'bears' below).
You need a car otherwise you can't get to the roadheads, or anywhere else for that matter. Nobody does so much as walk to the shops in America.
October sounds pretty late season to me, and most campsites will be closed (Tuolumne Meadows closes end Sept). People we met on our trip out to Mt Clarence King in early Sept told us that the average date for snow on the Kearsarge Pass (our way in and out) was 17th Sept. Getting caught out by bad weather on the wrong side of a pass could be a real adventure, and if you need help then a (long) walk over it and down to the nearest town may be the only way to get it if the roadhead sites are closed. I really suggest that you do some research to find out what is still open and accessible between the end of the summer season and the resort skiing season, and what the weather will be like.
You must bear-proof your food. A portable bear-proof box placed well away from the tent was the way then. Maybe they will be sleeping by October, though; something to check.
You will almost certainly need a permit for any overnight wild camping. The rangers carry guns, by the way, so best to conform to the rules.
There may be a 'pack out your poo' rule, depending where you go. Where necessary then kits for this may be provided at the start of the trail. Gets pretty whiffy after a few days!
The mountains are Alpine scale so some acclimatisation is necessary. Don't think of high summits otherwise. We came across people who thought they could do it, and they couldn't. However, bearing in mind the time of year and the potential for bad weather then you may not get anywhere high anyway.
Secor's book is good but think of it as all the Alps in one volume. The route descriptions are basic and a nose for the correct line is essential whether hiking or climbing. It's too heavy to carry so copy the pages you need.
If I were to say only one thing then I'd say go in the summer. The weather is usually sunny and that white granite is just super to climb on.